I have my two adult children booked on a Condor flight (SFO-FRA) that arrives at 5:10 pm on a Friday. I need to get them train tickets to Koeln (Cologne). There's a good option that leaves at 6:17 pm giving them a little over an hour to make it. They are flying Premium Economy so will be fairly early deplaning, and then they'll need to clear passport control, get their checked bags, and get to the Fern Bahnhof. They both have German passports and are young so running a little is not a problem. It's been a little while since I've flown into FRA but I can't remember ever having to wait a long time for bags, but I also have never flown Condor either. Thoughts? Maybe go for the 6:42 train?

I had a similar connection time (70 mins) earlier this year, also with an inbound Condor flight. Didn�t have checked bags and was quite familiar with the route to the Fernbahnhof. Unfortunately Condor was delayed, and my connection shrunk to something like 30 mins from doors open to train leaving. I did make it with 5 mins to spare, but next time I will either book a flexible ticket or book a later train and just plan on having lunch/dinner at the airport. So personally I would book the 6:42 train. I really hate sitting on a plane for 10 hrs knowing that my connection is very, very tight.

Make sure they know they can use the automatic immigration gates with their German passports. And that they understand the difference between the two train stations (Fernbahnhof and the one for the local/regional trains).

I'd second the idea of a flexible ticket.

Bag delivery can be highly variable theses days. Sometimes "normal", sometimes delayed, sometimes you're left wondering if they are ever coming.

Flexible ticket of course alleviates the problem, but they cost quite a bit more. Is there a price break for buying a flexible ticket early? I assume they are the same price whether buying now or upon arrival.

A Schnupper BahnCard 25% will reduce the price of the flexible ticket and will probably pay for itself with a return between Koln and FRA. Look into that.
But otherwise either book a late train or a flexible ticket, 70 min connection is very tight and it can easily be missed.