help!what is the english name of 波波池?

波波池=ball pit,
Most of the McDonald restaurants in the USA
have ball pits for kids to play inside.

Are you talking about Ball Pit?
http://images.google.co.uk/images?svnum=10&hl=en&q=ball+pit

In UK, sometimes it is called Ballroom (e.g. in Ikea).
http://www.ikea.com/ms/en_GB/about_ikea/store_experience/ikea_for_kids.html