LOL..... thats easy, Mac & Cheese! Hotdogs & Hamburgers. Let them make their own hamburger patties. And take fixin the buns away from Momma's, let them do it. They love it, they get filthy doing it and they'll all love it.
Don't over think it or try to impress kids. Do you have a ice cream machine? Its the simple pleasures they'll remember you for. Water balloons, make 'em up ahead of time? What about a dozen or so cheap squirt guns? They don't have TG&Y's or 5 and dimes anymore, but you know what I am talking about.
Take 5 or 6 big watermellons and just let them bust 'em on the drive and dig in with both hands.Give the hostess a couple to carve up afterwards and make pretty for the adults. Then hose 'em all off, the kids that is. Wait till after the S'Mores...... LOL
KISS and you'll be the coolest Uncle! Don't ask the Mom's premission, just appoligize when they complain. LOL Its much easier that way and hell, let their husbands listen all night long about it, you are the fun uncle! ROFLMAO The one that on their birthdays they want to open your present first?
All the great Pooh Pah's keep it simple! Let the adults bring the adult foods.