Even if it's not a hot summer in Zagreb (which is usually a fact in summer) there is no way your kids would say no to splashing in indoor pools at any time.

Not far from Avenue Mall, Utrina pools houses not only a big Olympic-size pool, but also pools for kids and a dive pool also. Swimming lessons and various programmes take place here as well, both for children and for adults. There is also a wellness center and a cafe.
Svetice is one the newest and most modern indoor pools in Zagreb, located next to Maksimir Stadium. It consists of a big Olympic pool, a smaller warm up pool, and a children’s pool. The 10,000 m2 Svetice complex also offers a wellness programme.

Prices for children 15kn, adults 20-25kn depending on whether it's weekend or holiday; check working hours on their websites.