I run pre-school music classes as an additional activity to instrumental teaching. I'm an independent operator, not a franchisee, so can only give a one-sided view of it from that point of view. However, I do know that some franchises are international as opposed to national, so if you want to go that route, then there are options. Kindermusik is one that operates globally. Not sure about signing groups though.
You've thought about most things so far.
Venues are one of the most important and you can source outside of the usual village/community/church hall - try sports clubs/pavilions as well, and groups similar to Guides and Scouts which may have their own premises. Some groups run in soft play centres which seems to work for them (although I think I'd probably rather sit naked in the street watching my feet burn, than run classes in any of my local ones!).
There are lots of resources on the web for ideas and themes for running classes - if you search for 'pre-school music lesson plans' or similar you'll come up with some interesting sites. Or if you're looking at particular themes, such as weather you can search for 'weather songs' and you'll lots of hits.
Sourcing instruments can be expensive - if you buy in bulk you may be able to negotiate wholesale prices. I have UK suppliers whose details I can pass on if you like - might not be so useful if you're not in the UK though!
If you've got any other specific questions then I'd be happy to pass on my experiences - I've been doing this for years, so have learnt a lot from my mistakes!