My 4 yr old has the same problem at the playground sometimes.
"Daddy, those bigger boys don't want to play with me (sob, sob)."
I tell him, "It's okay. Just because you want to play with them doesn't mean they have to play with you." Then I add, "You can play by yourself, find somone else to play with or you can play with me."
At which point he's already forgotten all about it and is on to the next thing. "Daddy, what? Look a doggie! Can we buy a balloon?"