No matter the age, all children can explore at the National Army Museum's Kids' Zone.
The Kids’ Zone is free and features a forest, a castle, construction, reading, art activities and board-game areas. While the Zone is tailored for under 10's, there's a soft play area for babies. Set in the fashionable Chelsea neighbourhood, you may even be able to squeeze in a quick shopping trip.
Kids' Zone at the National Army Museum
Royal Hospital Road, Chelsea, London SW3 4HT (further directions here.)
Nearest Tube: Sloane Square (District and Circle Line)