Eat early or late, ideally say 11 or 2:30 for lunch, if they can to avoid long queues. Use the mobile ordering if they can work it out (it should be fairly intuitive if they use it elsewhere). They can get cups of iced water from the collection line of all QS places, they do not need to wait in the ordering queue to get it.
Keep the kids hydrated and fed - they can bring snacks in. The last thing they need is a meltdown because they’re hungry or thirsty.
If there’s rides the older one wants to do which the younger one cannot, a couple of adults can wait with the younger one, and then ride with the older kid right after, using the FP line. They just need to ask the CM for a rider switch before the first group enter the line. Use it with FP or stand-by, it’s good for any ride where they offer an FP.
They can book additional FPs after they use up the first three. Show them how to do that.
Tell them there’s lots to see between rides. The wishing well, TSI for the kids to run about, the play area in the Dumbo tent, look for carved animals in Adventureland for a rest and snack or waiting for others. Use CoP and the People Mover as a rest for tired legs.
Tie something onto their stroller so they can find it again in the stroller parking. Streamers, a balloon, even a luggage tag, anything to make it stand out. And ake sure any favourite teddy is secured in a backpack, not left loose at the bottom of a stroller.