You should be able to make FP+ for anyone in your
MDE that has valid ticket media linked to their profile. That being said, you need to list everyone on the reservation who is staying in your room regardless of age. If you have tickets for all 3 of your kids, that leads me to believe they are all at least 3 or over which means you cannot be in a room that has an occupancy of 4. Everyone has to be listed, otherwise, those not listed cannot utilize Magical Express, EMH, will not get a Magic Band, etc. Not to mention it's against WDW rules.