What about making (or buying) some yummy chicken salad or tuna salad and then having some fun breads and bagels for the moms to build sandwiches. You can have the kid food you mentioned too or just do cold cuts for them. For parties at my house I usually have a few kid faves but cater mostly to the adults b/c kids don't eat that much especially when they are playing with a bunch of kids! I have done shaped grilled cheese which are really easy to make ahead of time. I buy the square Peppridge Farm bread and use a cookie cutter (you could do a sun or something else summery, they may even have a graduation cap out there!) and cut out the bread. I put cheese on the inside then I butter both sides of the bread and put them on a cookie sheet and put it in the fridge. Then right before you need them throw them in the oven for 5-10 minutes, flipping once and viola...you have fresh and fun sandwiches!
I've also found that making quiche for my parties is a big hit. Generally moms love them and you can make them and fully cook them the day before. I warm mine up in the oven so they are heated throught and then cut and serve. You can have a green salad to go with and some fresh fruit that both kids and adults will enjoy and you're set! Also muffins and breads are good too. I like your idea of a spritzer drink and homemade cookies too!
I'm a lot like you...I like to cook for my guests when I can and make new and fun things, but I've found the more you can do before hand the better! Have fun!
Just reread your question and saw 20 adults, you can do some egg casseroles that are just as good as quiche and a little easier for a big crowd, or go with frozen mini quiche as someone else suggested. I think at 11am you could easily do brunch or lunch and have it go over well!