Tuesday's Child IS alright, and they had a place at Macclesfield, which I assume's still there. I'm sure you can google details.
The one thing your child has to be, is well behaved. Shoots often take a while, and there's a lot of hanging around. The one thing that studios don't like, is when children start wandering round all over the place, or get tired and start to cry. Unfortunately, this can happen with very young children, so quiet, happy ones stand more chance of being picked. Also you have to be prepared to turn up when asked - not always easy if your child's of school age.