Two schools of thought here.

On one hand, giving them the upgrade on the way to MCO builds a little "excitement" for them, especially if they've never flown first before. So you start your vacation with a sense of lavishness (even though we all know it's nothing spectacular per se).

On the other hand returning home is always a downer and half time time after a week with the kids I feel like I'm traumatized, so the benefit of boarding first (twice) helps make life a little easier.

(Granted, my kids are 4 and 6 so any extended vacation has a 50/50 chance of being stressful. LOL.)