Kyoto in the summer months can be hard going if you are travelling with kids.
I have lots of indoor and outdoor suggestions for travelling in Kyoto with kids here but if you need somewhere with a splash park or shallow pool for kids in Kyoto, I recommend these spots in Kyoto:
Umekoji Park (the park next to the Kyoto Railway Museum) have a waterpark for kids set up in summer months (usually mid-July until late September.) Reservations required via the Viva Square Kyoto website here. Prices are around 1000-1300 yen per hour time slot.
Another alternative option for Kyoto with kids in Summer, taking a walk down to Kamogawa river and walking across the stepping stones and playing in the river.