People worry too much. DS has been playing unsupervised at NappiesGalore's since about this time last year... she has "grounds" - all enclosed, but climbing and falling hazards galore, and you have to go and FIND them rather than glancing out a window. Oh, normally with a 5, 4, and same age child although now it is just the same age one as the others are normally at school.
Originally there was a nanny who kept half an eye on them, but she left in the summer and I'm just used to letting him run now.
They both love the freedom there though. They come home filthy as hell, but so happy.