Bless you and DH, Trudy! You've taken on quite a task and those children are very lucky to have you!
I don't know much about foster care laws. But please check out all possible resources. I've heard that sometimes there is financial help available that GPs aren't aware of. If you're actually working with the foster care system, then I think they have to cover the same costs they would for any other foster parents, but I'm not sure.
Also, I hope you check into any possible counseling services, as I'm sure one or more of the kids will need it, after whatever they've been through with their parents.
Best of luck to you all!