I should probably mention that Perenelle Flamel would volunteer to foster any children (her family has a Hogsmeade home). @"Rosie Binns" would also offer her home (her husband would probably love the positive light the charitable act brings, though that's not why she is open to the idea).

Edit: I forgot to mention, the Ruskin household would be willing to act as a foster family (if the Ministry would approve, given the half-vampires on the property during summer).