It is mapped already as a "speelbos". The wiki says leisure=playground + playground=forest and https://www.openstreetmap.org/way/948099180 has exactly those tags. The rendering makes it look like a regular playground, but we don't tag for the render. If you think the rendering or the data model isn't great, that's something to discuss elsewhere (check the channels here: https://openstreetmap.be/en/contact.html)