If your child asks other children, "Can I play?" the answer will be "No!" about 60% of the time according to researchers. The other children are already engrossed in their play and it's easier to say "no" than to figure out how to include someone else.
A better question to teach your children to ask is "How can I play?" This invites the other kids to figure out a way to include, rather than exclude, the new child. It's a simple way to rephrase the request and increase the likelihood of being welcomed into the play.