Hi there,
Sadly, Prague is not so big on having amusement parks. In spring there is a fair (Matejska Pout) with roller coasters and rides, but that's there only in the spring season starting from February until about April.
Even though , there are no amusement parks, Prague does offer quite a lot of water parks, with slides and different types of pools. I would say that the most popular one is Aquapalace.