Rules.... rules... rules... remember folks, these are rules, NOT LAWS! Moses did not come down the mountain with laws about back to back or nested trips carved onto a stone tablet.

Several decades ago, we were presented with airline "de-regulation" but what we really got was airline self regulation!

Oh wow, I never knew this was against the rules. I had this many times when taking a weekend off to some place other then home when working Monday thru Thursday at client sites.

No. They aren't rules. They are contract terms to which you agree when you purchase a ticket. If you don't like the law, that's tough. If you don't like the terms on which a service is offered, don't purchase it. Don't give it a moment's further thought and move on.

Remember, the downside of regulation was that there were basically two fares: F & Y. Then a couple of specialty fares such as round-trips and the like. Baggage allowance was 66 poounds in F and 40 in Y.

But, no fixed (not changeable/refundable). So, no games worth playing, no hidden city, nested and the like.

It's not against the rules to have a "nested" ticket to a third location.

Shouldn't be a problem. I did it a few times on Delta when my children were younger.
If the system won't let you book online call and ask for a the phone reservation fee to be waived (you are eligible for a waiver if the website won't support this).
Your last recourse is to book itinerary B with a different airline (DL can't restrict you from doing business with other airlines, trying to do so will be infringement of anti trust laws).

Happy Travels
DLP

Without knowing the city that you refer to as XXX it is impossible to give you an accurate answer. If XXX requires a certain minimum stay, etc. then the answer is different than if that is not the case.

Also, you don't NEED to book 3 round trip tickets then you separately from XXX to MSP to XXX. You can book the other two in one reservation, you in one reservation for all FOUR flights then fly with them. Being in one reservation alone does not require UM status - only if an adult is not flying with them. I know this to be a fact and have had friends do just that. The only differences are (1) if you book your ticket as one ticket for all four legs, you have no issues at all if DL prices it like you expect, (2) most likely you save a few dollars in taxes.

The following is from delta.com:

Prohibited Ticketing Practices
Our fares apply only to the specific itineraries for which they are quoted and the restrictions that apply to our discounted fares are an essential part of our contract with you. These restrictions make it possible for us to offer these discounted fares.
Failure to comply with applicable fare restrictions, circumventing those restrictions, or misrepresenting your intended itinerary are all violations of our Contract of Carriage.
While not an exclusive list, the following ticketing practices are prohibited:
Back-to-back ticketing�combining multiple overlapping round-trip tickets to circumvent Saturday or other overnight stay requirements.
Throw-away ticketing�use of discounted round-trip excursion fares for one-way travel.
Point-beyond ticketing�use of a fare published for travel to a point beyond your actual intended destination or from a point before your actual intended origin.
In such cases where there is a violation of our Contract of Carriage, we reserve the right to:
Cancel the remainder of the itinerary and confiscate any unused flight coupons.
Refuse to board the passenger or check baggage.
Charge the passenger for the difference between the fare paid and the fare for the passenger's traveled itinerary.

As others have said, this would be back-to-back ticketing if the OP's proposed itineraries circumvent fare requirements.

It's probably best just to call Delta and ask. The OP makes a valid case for wanting to be on the same PNR with kids, especially in case of IROPs. That seems a reasonable justification for booking two itineraries.

So I booked a dummy ticket and it looks like to get the fare I would like to buy (both itineraries), the rules are:

RESERVATIONS/TICKETING RES REQ 14 DAYS BEFORE DEPART
TKTG WITHIN 1 DAY AFTER RESERVATIONS
OR AT LEAST 14 DAYS BEFORE DEPART
WHICHEVER IS EARLIER
WAITLISTING NOT PERMITTED ALL SEGMENTS
FURTHER RESTR APPLY SEE TEXT RULE
MINIMUM STAY FOR TRAVEL ON/BEFORE 19NOV 11
BEGIN TVL MON/TUE/WED/SUN RETURN AFTER 3 DAYS
AFTER DEPARTURE FROM ORIGIN
FOR TRAVEL ON/BEFORE 19NOV 11
BEGIN TVL THU/FRI RETURN FIRST
FURTHER RESTR APPLY SEE TEXT RULE
MAXIMUM STAY 30 DAYS AFTER DEPARTURE FROM ORIGIN

It looks like I need to stay 3 days at least to get this ticket, so if I fly in on a Thursday night I should be able to leave Sunday? Or Monday? And still be within the contract. I don't actually know what Delta considers "3 days".

I have flown with my kids on separate PNRs about 10 times. 4 of those times we've had irops and things have gone awry - the last time was in 2009, when we had a mech delay coming from the west coast to MSP and the kids were placed on a route via DTW, me via SEA, and my husband nonstop. We missed two potential departures because the agent helping us had a lot of trouble trying to undo what the computer had helpfully done (one kid was on an infant fare, which complicated things). I am really leery of splitting from them until they get older.

Sunday. But the fare rules seem to be different for Thursday/Friday departures (and some language seems to be missing).

Technically it is "back-to-back" ticketing and technically since you are returning 1 day after departing, you are circumventing the rules. Will DL catch you? If they do, will they do anything? The answer to both questions is "probably not." No matter how DL defines "3 days", Feb 1 to Feb 2 is NOT enough . Neither is Feb 14 to Feb 15.

I don't think it is back-to-back since the 3 day minimum is no longer in effect (expires 11/19/11). Probably have to look at the full rules to know, but these look incomplete. In any case, it's highly unlikely that OP would have a problem with what he would like to do.

Other than clicking the link marked "Fare Rules", how do I find the more complete version? I clicked on another link that said "View Full Fare Rules" and just got a page with a link to a page of definitions. There are several referrals to a "text rule" but I can't see where that is.

The fare code is TD14A0SD if that provides any clues.

If I book as one itinerary (as fti suggested), it actually prices out about $100 cheaper. I may throw PNR caution to the wind.

I have done it before on Delta, and had no problems, when I flew to Tulsa to accompany my aunt to Birmingham. The last time I did that was six years ago, and I flew NW on a round trip for two weeks, and sandwiched a AA roundtrip TUL-BHM with the aunt. I earned MQMs on AA and NW. I have Million Miler miles with Delta as a result.

Using a different airline changes the equation completely and is not comparable to the currently proposed scenario.

Again, the issue is whether you are trying to circumvent the fare rule. Try booking the alternate set of tickets without nesting and see if the fares are the same . This should give you peace of mind.