Addition to the previous water place, Shioiri Park, there is another one we've been to with my son.
It's Izumi Park in Chiyoda-ku.

It's called "Jabu jabu Pool" and "Jabu jabu" is the sound of water splash in Japanese.

Unlike the one at Shioiri Park, Izumi Park one is warm water.
So, the kids won't get cold after playing.

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It's opened from July 13 to Sep 1, 10:00am to 4:00pm except for 12:00pm to 1:00pm.
It's a public park and of course it's free.
The place if for the kids under 6 years old.
There are always some facility staff watching the place.

Once you enter the pool area, you and your kids will be guided to take off the shoes.
Kids cannot play with a diaper on.

There are lots of buckets and watering pots kids can play, so you don't have to bring anything.
But if you want to, of course you can.

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There are some benches with roof beside the pool, so parents can watch kids and rest.

Again, kids will take a long nap after playing with water for some reason.
So, letting them play here will give you a bit more quiet time.

And here are the other parks with kids pool in Chiyoda-ku.

- Togo Gansui Kinen Park
- Chidori Gafuchi Park
- Kanda Jido Park
- Horin Park

There are lots of water places kids can play for free in Tokyo.
I'll share with you when I visit.