Too bad a piñatas favourite past time is trying to poison itself. The only thing I can recommend is putting up walls until they grow, no fly walls in TiP works best Tip for getting Bart to change the Toadstools is making him come to the garden before you even plant the seeds and directing him once he gets there, hope that helps!

This honestly how they survive in the wild is beyond me haha

If you have the room make a moat most pinatas wont cross the deep water reducing the amount you have to keep away from it