There are two answers to this. First, if you’ve bought a tower playhouse with a slide, you could use the underneath space as storage or even a garage for kids' cars. On the other hand you might think about installing a foundation or base> under your playhouse. We’d advise against building your playhouse directly on to grass as it can lead to undue pressure on the structure as well as water ingress, damp, and rot.  If you wish to fit a base under your playhouse, then a timber sub-floor, existing patio, paving slabs, concrete slab, or gravel and plastic foundation all work. For more, read this post on preparing your garden for an outdoor building.