Listen first
Ask about a specific break, not about their whole personality. “Why don’t you just go and play?” adds pressure.
Name an adult
Who can they speak to if they are left out? School and parents carry protection, not the child alone.
Choose one small attempt
One sentence for asking to join is enough as practice. New friends are not a coaching promise.
Take exclusion seriously
Repeated humiliation is not a friendship problem. Then safeguarding — and the page on bullying if that is what is happening.