the kids menu is a set one, you pay a set price for, and I think its really bad value for money to be honest.
From the a la carte menu you can order what you like, but it does need to be at least, as you say, starter and a dessert, it needs to be a meal.
Lots of folk (me included) order a couple of pizzas from the a la carte menu to share between 3 or 4 of us at Cafe Mickey. Add some soft drinks and the bill shouldn't come to more than about £40 total.
I've done both lunch and dinner at Cafe Mickey, its good at lunch to break up a busy day in the parks, and its good in low season when the parks close early, for dinner. So its entirely up to you which would suit your family best.