Birthday party at a park food ?

So cook things ahead of time- you could roast a chicken or two (or even easier, buy a couple of frozen bags of chicken tenders), and slice it up, have salad or cut up vegetables, and some little pita pockets. Make some jelly sandwiches for the kids- no peanut butter, in case anyone has allergies, maybe some almond butter or sunbutter instead. Use a cookie cutter to make them into a couple of cute shapes.

That, a cake and a bunch of paper plates, napkins and juice boxes should do it.

You might also get some bottles of bubble stuff - you shouldn't have balloons at a park, but bubbles are fun!