On a hot summer's day, I would definitely recommend a day trip out of the city to Divoka Sarka and in particular to this outdoor swimming pool.
The tram ride is about 20 mins out of Prague and then, if going direct to the pool, it's about a 1km walk along a wide tarmac path through some very pleasant scenery alongside a stream (and with a fair few benches to stop for a snack/lunch along the way). The children are now all old enough to walk, but other families seemed to be having no problem with buggies for younger children, so it's more accessible than it might seem.
Once there, the pool is nestled in the valley and surrounded by a good amount of green space, including a mix of sun and shade. The staff understood enough English to sell us the right tickets, which were good value for a family of four, especially given you could spend all day here. Hiring a private cabin to change and keep bags and belongings safe was definitely worth it too. The water was admittedly cold (think of it as character-building!), but it wasn't too long before we acclimatised and the children were enjoying plunging into the pool from the small slide.
The place was busy enough to have some atmosphere, but not in danger of feeling overcrowded.
There was also a cafe on site - we only made use of it for some welcome (and not over-priced) ice creams to enjoy as we made our way back to the tram.