this may be helpful (although I do know it is from the dark side) but not many huns on this discussion board. There are some messy play/arts & crafts people on there who have lots of experience - takes a bit of reading but useful info is hidden away there.
Briefly, regarding tax, NI, insurance etc - if you're looking to set up as self-employed then you have 3 months from setting up the business to register with HMRC or you will incur a £100 fine. (However, if you don't make any money, you don't pay tax.)
Public Liability insurance is definitely worth having for peace of mind - mine cost around £85 for a year with £2 million cover.
At present CRB checks aren't necessary if you're not working with the children without their parents being present, but I'm assuming you would have one anyway being a Brownie leader. A CRB check is useful to have because some church halls will not hire to people without having one.
Most classes where I'm based (South East) range from £4 up to £6 an hour - these are for music, toddler gym/activities etc as we have no messy play or craft groups. Several have started but not continued - not sure why really as I am in Yummy Mummy commuter belt, but it hasn't taken off here. I would imagine, because of your demands on equipment and having to constantly renew your resources of paint, glue etc you would need to charge more than, say a music class, which has fixed equipment costs.
Class size for music groups is usually around 14 children max, but perhaps with crafts, where you don't have the same noise levels, you could have up to 20 depending on space.
I read the other thread as well, and I agree with some of the comments - I did crafts with my 2 DCs (although found it horrendously stressful) and then once they went to pre-school they did all the necessary messy stuff there. But then I only took my eldest to baby music, and just for a term because it was so bloody awful - that's what inspired me to set up my own music classes as I thought I could do much better myself!