No more balance on the block...

Well, the honeymoon is over.  Earlier this year we moved into a new house and our 2.5  year old daughter fit right into the spot left by the former owner's two year old.  There are a lot of great kids on the block, and right next door we have a new best friend and playmate who is 4.  The girls asked to play together almost daily, and my neighbor and I traded houses for the playdates so we each had time alone when we needed to get something done or time together to chat as the girls played, it was perfect.  Then this summer... new neighbors.  They are lovely, don't get me wrong, and I sort of feel a sense of betrayal as I type out these feelings, but they have a 5 year old daughter who has upset the friendship balance on our block.  Mainly for my 2 and a half year old, but also for me.  My daughter loves the new neighbor across the street and we've had successful playdates with her.  But the 4 and 5 year old girls are close in age and have become instant best friends.  When they play one on one with my daughter things are still good, but increasingly they want to be together, and while they are generous and happy to include my daughter, she is quickly left behind and tears soon follow.  My daughter is feeling her first pangs of rejection....and I can't help but feel them too.  My closeness with my neighbor is being tested, as she now has two outlets for socialization and breaks, and her daughter would rather play with her new same aged friend.  We try to have all three girls play, but I end up having to play constant referee in this scenario so I either struggle alone with all three girls at my house, or miss the entire adult conversation while my friends chat without seeming to notice the change.  I am struggling to figure out a way to communicate my feelings about this to them, a part of it feels like a junior high popularity contest or something.  I am sure things will continue to evolve as the girls, and all the other kids on the block, grow and change.  But for now...I definitely feel like we are on the outs and I am bummed!
How have others out there handled this?  Have things ebbed and flowed with the kids on your block?