The large (Groot Kinderbad Amsterdamse Bos) and small (Klein Kinderbad) pools are filled with water from May to September, on days when temperature is above 20 degrees. In September, the large pool is open only on weekends.
Check here information about swimming pools in Dutch https://www.amsterdamsebos.nl/attracties/kinderbaden/.
Getting there: coordinates of large pool 52.317003, 4.823328, of small pool 52.324493, 4.849352. Parking lot for the large pool 52.316350, 4.817886, for the small pool 52.321641, 4.849344. The nearest bus stop near the small pool is Amsterdam, Amsterdamse Bos.