Coram's Fields
A park which is run as children's play area. Facilities include a pet's corner, lawns, sports pitches and a nursery. No adults are allowed to enter the park unless they are accompanying children. All visitors are welcome to join the local kids.

A park which is run as children's play area. Facilities include a pet's corner, lawns, sports pitches and a nursery. No adults are allowed to enter the park unless they are accompanying children. All visitors are welcome to join the local kids.
Check the official site before travelling; hours and prices can change.
More parks

Hampstead Heath
320 hectares of wild north London — swim the ponds in summer, climb Parliament Hill for the skyline.

Greenwich Park
Royal park with the Observatory at the top — best skyline view east of central.

Regent's Park
Queen Mary's rose garden in June. Primrose Hill at sunset, year-round.
Source: wikivoyage