The German Shepherd rescue I volunteer for will not place certain dogs if the adopters have smaller children. Most of the time it is because the dog has jumping on people issues but sometimes the dog has food and toy possession issues.
The German Shepherd I am fostering cannot be placed with families with smaller children or people with disabilities because he is too strong for them (not a jumper or possessive).
You are an absolute doll for fostering and working with an organization that helps dogs get adopted. I know one elderly gentleman that adopted from a GSD rescue. The two of them were best buddies from day one. By buddies, I mean they even had ice cream together each night. Both of their lives improved via the rescue/adoption.