L.C.
Hi K.,
I agree with Leslie that in general we don't hang out with people that, for whatever reason, we don't click with so it seems unreasonable to ask your daughter to do it. But then my heart goes out to the little girl. I can't remember how long it's like this, but child development specialists say that young children understand the concept of kids that are younger than them but not children that are older than them, so while the little one wouldn't want to play with an 18 month old because that's a baby, she doesn't see any difference between her and a 9 or 10 year old. Plus she's probably a little dazzled. I also think, if you know or suspect your daughter isn't being super nice and leading the group that direction, you should discourage it and use the situation as an opportunity to teach her about empathy and compassion.
What if you had periodic specific playdates that were for everyone to do together or the girls could do so that little one could be included and then it's over and everyone goes home. Things like manicure/pedicures sessions, makeovers, crafts, waterplay outside. You could sit down with your big daughter and explain that there are going to be these playdates that everyone does and then there are the ones where she will be with her big friends and not have to entertain a 4 yr. old. You can tell her why, and what your expectatioins are. The better she mentors the 4 yr. old, the more opportunities she has to be with the big ones because it shows responsibility and kindness. Plus it's good experience for when she is old enough and wants to start babysitting.
Then little girl's mom will feel better, little girl won't feel left out all the time, 9 yr. old won't feel put upon all the time, and you won't be encouraging what is maybe some not so nice behavior because your daughter is frustrated.
Hope this helps,
L.