My two kids (8 &10 yrs. Old) have two best friends who are brother and sister. Their parents never seem to want to get them together to play. We invite them everywhere, and they come with us, but they never reciprocate. My kids are noticing and they feel hurt by this. What should we do?

What do you do when your child has a best-friend and they always want to play together and continuously ask to play, yet the parent of the other child will not let them play?
Do you:
A. Approach the other parent and ask what the problem is?
B. Let it go, and tell your child to "find another friend"?
C.None of the above.
D.All of the above?
Signed:
Sad for my son... :(