This requires many asterisks, as once you hit any appreciable size of "giving out food" you tend to hit tons of local ordinance about food safety, permits, and just general distrust of directly interacting with other people's kids at a playground (depending on the age we are talking about, but since we said playground, I'm assuming pretty young).

> giving kids free food is not illegal

It’s not. But if you’re giving a kid “100” burgers “in one sitting” without the parent’s explicit sign-off, you are probably liable for damages.

If the food contains heroin, it's illegal. There's a line somewhere.

If you believe that, go set up a "Free Candy!" stand at a local playground and see how long before the police show up.