Some other advice
1) IF the seatbelt does not fit over the carseat, the FA have seatbelt extenders
2) IF the carseat seems tight, lift up the armrest and then put the seat in. You can then try to put the armrest down.
3) Check you bags and only take the min. We back formula but use the containers where you only need enough for 1 1/2 times the flight (for delays)
4) Agree on having them eat at take off and landing. Have some juice ready if they are not hungry (we do not give juice to the little ones, but during take off it is very useful)
5) Try, Try Try to keep them up and then let them sleep on the plane
6) Do not hesitate to ask for help from the business traveler who is traveling light. Some one will most likely offer to help you (I do whenever I see a mom and my wife has always had someone offer - just never the airline employees)