I have an 8 year old girl and a 4 year old boy.
I have play-dates all the time for both of them.
Either together or separately, per friends.
If my son, the 4 year old wants to play, he does. My daughter does not mind. They ALL play with him.
If my daughter's friend says she doesn't want my son to play (with them), I tell ALL the kids, that "this is Jane's brother, he is family, so be nice."
My daughter, sticks up for her brother.
I DO NOT LET THE OTHER KIDS TREAT MY SON THAT WAY. It is my house, our play-date, and I teach ALL the kids, to be nice.
I ALSO teach the older kids, my daughter's friends, that YES, her brother is younger... SO THAT MEANS, be respectful of that fact. And sure, he cannot necessarily do everything they can. But he can be included and treated fairly. They are old enough to understand, that younger kids, are younger.
I also explain to my son, that that is his sister's friends, and HE knows them too. But sometimes they want to play alone. Its okay. If he has trouble, with them, to tell me.
I also explain to him, that not 'everything' his older sister does, is his age activities. He understands that. Even at his age.
I do not let any kid, mistreat my kids or each other. When I have play-dates at our home.
Kids are at our house all the time.
all the best,
Susan