Sorry you are dealing with this. On a positive note, though, I think there are some things that indicate that this may not be as bad as it may seem.
--It sounds like you are saying the boy told his mom what happened (you said your dh sent him home). That is positive in that he did not lie and actually chose to tell his mom.
--The mom came over and had him apologize. This means that she recognizes it was not okay and is not avoiding dealing with it. I actually think she handled it very well.
--Although the age difference is significant, this can fall into the realm of normal childhood curiosity..it also may not--that is not known.
I think I would call the mom on the phone, thank her so much for addressing this directly with her son by having him apologize. Get together with her and see what she thinks about what happened..she sounds pretty responsible from the limited info we know from your post. I think then I would just say (depending upon what she says) that the kids should always have supervision when they play together. I would make sure either you or dh is providing this, even if the mom agrees.
The other thing I would take into account is how your dd is feeling about this. Is she fine and feeling it is no big deal. Or is she acting differently/not wanting to be around the boy? I would say more about this, but have to go so will write more later if no one else addresses what I was going to say!
lucky single mom to 20 yr old dd and 17 yr old ds through 2 very different adoption routes