I think you need to discuss that with the breeder. I doubt there's a one size fits all co-ownership arrangement, so if there are things you'd like to include you should see if s/he's willing to do that. If the breeder has done this before they may have a standard contract, but that doesn't necessarily mean everything is non-negotiable. You won't know unless you ask.
I will say that taking on an 8 week old puppy with a 6 month old already at home will be a lot of work!