Great idea! We are just making playgrounds and often recommend a gravel base with good drainage. The specialists from
https://www.siteprep.com/ make such pads — first they level the area, arrange drainage, then put a layer of crushed stone, and on top you can put a soft covering, for example, wood chips or artificial turf. It is safe, the water drains quickly, and the playground remains dry. Concrete is also an option, but it is hard and can be dangerous if you fall. If safety and comfort for children are important, then soft gravel will be better.