That is a difficult situation for sure.
That seems like a tough one. I guess I would be tempted to talk to her at a 5 year old level and tell her that as happy as you are that she visits - that your son gets upset when she leaves so soon or doesn't want to play. Then you could ask her what types of things she would like to do - and see if you can find something that they would both enjoy? If you angle it so she knows that you want it to work out - it be beneficial for everyone :) Or.. I'd talk to her parents... and let them know that you feel like the visits aren't going as well as they could be - and ask for suggestions for her favorite things to do :)