I would be more concerned about taking them in at that age if you had other cats/pets. Unless and until they're able to get fully screened and vaccinated, there is always some chance I suppose that they may have something other cats could contract. This would be special concern since the mom was a stray. In saying that, however, I'd note that I am not a vet; anybody who is or knows more about communicable disease among cats can correct me if I'm wrong.

Welcome to the wonderful world of cat ownership. They're amazing animals and companions.