Women Like Us is a good one to look at. They are a social enterprise themselves though, so people are more accepting of their model. When I used them previously, they charged a small fee up front event if they didn't find us somebody (which they didn't).
I receive their weekly jobs bulletin (always keeping an eye out for things I can do as a freelancer) and they seem to be succesful. They also offer skills/back to work training for women that have been out of the job market. Plus they are good at policy/press work, they produced a depressing report recently which said only 3% of part-time jobs offer £20K (full time equ) or more.
I think your idea works, but you need a hook. I am guessing price, the flex of not having an employee and the ability to buy a little time (rather than a full day) of a temp is a good one. I personally question how receptive local small businesses (unless run by a mum!) will be to messaging around the temps background, but they could be sold on the benefits above.
Mumsthatemp.com?
As you might know, there is a thriving 'mumpreneur' (I personally a bit
at the label) community, esp online & on twitter. Perhaps they could be the market you focus on, as they might be more open to the idea of giving mums some work? And growing businesses might need the sort of help you mention. www.mumsclub.co.uk/ www.mumpreneuruk.com/
Finally, I think you need a really good website, you could automate a lot of what you are talking about (registering candidates, taking job bookings) but you need to compete with brands like Women Like Us that have good web presence. I am guessing there are probably off the shelf recruitment website products out there that you can then customise.
Anyway, good luck with it all.