Owl, I can't understand your post.
Foster carers are not necessarily experts, they are just ordinary dog owners who want to help dogs find a great forever home. Any reputable rescue worth its salt is very careful about matching foster dogs to foster homes especially at first when you are finding your feet. There is also behavioural backup and lots of advice and support from foster co-ordinators at the end of the phone. As you get more experienced as a foster, then you may get asked to handle more challenging dogs or dogs whom less is known about. But only if you are willing/able - charities will work with foster carers to get the best possible match in circumstances.
Renting would only be a problem if your landlord didn't allow dogs (which presumably isn't the case since you already have one).
Why would having a disabled child be a barrier? I know masses of people who foster in all sorts of states of health - including wheelchair users. Two of my closest dog fostering friends are blue badge holders, and my own health isn't brilliant. Of my friends, one is up to foster no. 38 at the last count and the other acts as an emergency foster home for a local greyhound rescue when a place needs to be found fast. The only problem I can imagine would be if your child might be very noisy/shouty which might be a problem for some dogs. Some foster carers have DC, others (like me) don't. Most rescues are desperate for foster carers with DC as it's really good for assessing dogs before moving into a forever family.
Most charities/rescues have an assessment/homecheck process for foster homes - unless they specifically say "We don't want collie owning, home renting women with disabled DC " 
then you should pick up the phone or give them a call.
Channelling my inner Mrs. Doyle here (Go on, go on, go on, go on, go on, go on, go on, go on) 