Apart of the obvious: Prague ZOO, Letna park or Stromovka park, where they can run, ride and be entertain for hours you may need to slip in a short "play break" while exploring the city itself.
Luckily downtown Prague offers several nice spot, where your little ones can get the extra energy out and you can rest your feet.
Our favorite playground downtown - right next to the Charles Bridge. Slightly tricky to get there, basically look for directions to the Kampa Park restaurant and just before you enter their gate you find the playground to your right.
And this is the view you can enjoy while hanging out there.
Not far away on the Kampa island is another small playground with slides, swings and sand box.
Between Old Town and Lesser Quarter under the Legii bridge (most Legii) is Strelecky ostrov and man that is a life safer in the hot summer months. Small play area, but it is possible to dip your feet in the water there, as one side offers very shallow water. Plus it has a sand bank, so it looks like a tiny beach in the middle of the city. In summer - 10 degrees cooler than the streets, but you may need to share with few ducks and swans.
Further down you can find another island Zofin, which is also home to a playground, very sandy playground. In its neighborhood you can let your little train fans ride a small train round and round and round.
If you strictly insist on the very heart of downtown or have no other option, you are still lucky as there is a cute, but not shaded playground in the Frantiskanska zahrada, which is right next to the Wenceslas square. Literally. Right there.
And when it rains?
That will be a whole other post, but Lego museum, black light mini golf, great exhibition in the National museum or Mirror Maze...oh boy, so much to do!