They have a softplay there too x
If your little one is interested in helicopters or planes the Royal Air Force museum is good and free to attend. A little bit further but Birchen Grove Garden Centre is always lots of fun and good way to spend the day. There is a cafe with small Wendy houses to play in, outdoor playground, aquarium where you can pay to feed the fish and also Bear Town which provides fun imagination play for little ones (up to 7 years old). While there you can always do a walk around Welsh Harp (car park for it is just before the entry to garden centre).
Belmont farm! It’s in Mill Hill! X we love it x