I recently contributed to a local magazine about where to cool off on hot sunny days. Since the weather is sooooo lovely and warm right now, it may be helpful to list the various options here. Further suggestions welcome!
Kensington Memorial Park — Tucked away in W10, this park also includes an exciting and interactive splash pad with water fountains and sensors. It’s perfect for young and older kids alike.
Parliament Hill Paddling Pool— Hampstead Heath creates a picturesque backdrop for this paddling pool. Located within the Parliament Hill Fields Playground, children can choose to splash in the water or dry off on the swings. Nearest entrance off Nassington Road.
Victoria & Albert Museum— For mild water play and splashing, the courtyard at Victoria & Albert Musuem provides a decidedly breathtaking setting. Grab a drink or small bite to eat at the cafe whilst your little one dips her toes in the water.
Somerset House— Kids are free to run through the fifty-five fountains at this iconic arts and cultural centre. Entrance to the courtyard is free.
Serpentine Lido — Located in Hyde Park, children can choose between a small paddling pool and the playground. There is a fee to enter the Lido, which keeps the crowds at bay. If you're feeling peckish, a cafe is located below the facility.
Diana Memorial Fountain — Just south of the Serpentine in Hyde Park (and next to the Serpentine Lido), this is a mega popular spot on sunny days. Children run up and down the fountain, splashing their feet in the water. Arrive early to avoid heavy tourist crowds.
Also, Here's a link toLondon Baby'slist of top picks for outdoor water fun too.