I have 2 DD's. My older DD met a girl in Kindergarden. They became instant friends and have been best friends now for 4 years. The mom and I get along and they have a younger DD a year younger than my DD6. The two younger ones get along, but theirs also never likes to clean up and she will sit on the floor and pout and temper tantrum if you even ask her too. I am lucky in that the mom is way on top of it. Now, when the girls get together, we go to their house, that way mom can control it and what punishment to divy out. Maybe that is a suggestion for you, go to their house. If that isn't an option, and they must come to your house, limit the younger ones to a family room and tell them that they can pull out only one toy at a time, and if they want another, they must clean up what they already have out before they get another. After a while the other girl will get the picture. Or maybe you can arrange play dates at a park, or Chuckee E Cheese. You get the picture, just limit how much the younger one can pull out and mess up.
Unfortunately, I also encountered the Santa issue this year with both my DD's. They both still believe in Santa, Tooth Fairy and such. Last year at the Harvest Party at our church, one of the little girls from my church told my older DD that Santa wasn't real. She burst into tears. I found out what happened, I carried my DD over to the other mom and I told her to fix what her kid had done. I told the other mom; "You may teach your kids one thing, but we tend to let our kids believe and I do not appreciate your kids ruining it for mine." I agree that if you want to teach your that make believe figures don't exist then, you also need to teach your kids to keep it to themselves. Yes, my kids will figure it out on their own, but I don't want their imaginations squelched at the age of 9 & 6. They are only in that magical time frame for such a short time, and there is nothing like the sparkle in your own child's eye as they see that dream character for the first time. I think you handled it very well. Good Luck!