I cut out gluten and dairy for myself, and I figured it was likely to be good for the kids, but I was in no shape to do separate meals for me and them, so I was the driving force. That said, I saw enough good things in DD to make me glad we're GF.
Things I saw in DD:
stopped waking at night (she was 3.5yo at that point)
she *started* napping again, after no naps for >1 yr
she stopped wetting the bed at night
her tonsils went down to a normal size (this took about a month, it was the slowest thing for her)
her poop got firmer (it had always been fairly squishy), and she pooped less (from about 3x/day to 1/day IIRC)
We ate super-boring for a while, lots of lunches of lentils and rice, and eggs for breakfast lots and lots (still sorta burnt out on scrambled eggs for breakfast), but I eventually realized that a lot of our dinners either were GF naturally or could easily be made GF, basic stuff like roast (beef or chicken) + green veg + something starchy (GF grain or potatoes or sweet potatoes), lots of bean dishes are GF, stir fries w/o soy sauce (or with wheat-free soy sauce, but later figured out we're intolerant of soy as well, but they still taste good w/o soy sauce). And I eventually got a few basic recipes--I don't do a ton of GF baking, but I have one or two cookie recipes that are easy, I've got a bread recipe, a cake recipe (though some of the GF mixes are quite good), and a pizza dough recipe. I'm not the super-GF-baker, so I just wanted a few basics that work for us, and then I occasionally experiment.