Having used MN a lot less over the last year I can't work out how to search for a thread, I tried, honest!
The thread I mean was not long ago and I think it was Staryeyed who was looking for a gym or something for an under 5.
If she is reading this, I wondered if she is in touch with a group in Barnet called YASS! I just got their latest newsletter and, although not specifically a gym session, there are a few things in there that she might find interesting. Stary, if it was you and you CAT me I can forward it or post a contact for YASS here.