Hot dogs or pizza. Is what I’ve used to at all kids birthday partys. The last one we went to was happy meals. (But the kiddo had some rough medical stuff this past year so he got a super special birthday party).
Grilled hotdogs would be the best choice even if it wasn’t the cheapest. Most kids think hotdogs are a treat. I cook hotdogs in simmering water and then let the kids cook their own over a campfire for a real fun treat.