M.L.
Organized games can still be awesome with 20 kids... in fact it will probably make the party better. I know, I'm a professional party entertainer! You can set up a simple game of hopscotch, limbo, hoola hoop contests, relay races.
For one game, everyone puts one shoe in a big pile. Then. they have to stand in a big circle with the shoes in the middle. When the music turns on,. they go, find their shoe and put it on. Whoever gets the shoe first wins! Obviously, this is easier if the child is wearing sandals as opposed to lace up shoes, so you could even have separate games.
Another really fun game, is magical freckles. Buy round paper confetti or make your own using a 3 hole punch and colorful paper. Put the children in pairs and give each pair a bowl with paper confetti that is slightly dampened by a water spritzer. Then, on the count of go, one child has to put as many freckles on the other as fast as they can. When you call stop, they have to stop! All the freckle faces line up and the parents count who has the most freckles and that team wins. play it again so the other partner gets the freckles.
Do you have a theme? If so, play your games according to the theme. Do a scavenger hunt outside... lots of fun ideas, just google "outdoor children party activities"