I've never been able to combine two fare buckets on the same PNR. Even if you force the kids onto a separate PNR and link them (which would probably involve a $30 phone ticketing fee), you run the risk of the system splitting you if there's an IROP.

I just did the same thing (though two adults, two kids, two linked PNRs) to save a few bucks. Long story short, IROPs forced a rebooking, and we ended up on two different routes. Not the end of the world in my case, and I was able to fix it by getting the nice folks at the club to rebook us via a connection that technically didn't exist. When it is just one adult with kids, I just pay the higher fare to be on the same PNR.

Perhaps one of the smart folks on FT has a better solution.