L.,
This would drive me crazy too. I can't stand it when parents don't do their job, and think nothing of letting their work fall on others.
You're in a really sticky situation, and I guess you have to weigh the value of your relationship with your neighbors with the value of your personal space, etc.
On a very serious note, when someone on your property gets hurt, you are legally responsible, so to protect yourself against a situation that could greatly impact your financial security and that of your children, I would be a little more proactive with your neighbor about keeping her kids reigned in. If all diplomatic efforts fail, I would tell her that if arrangements have not been made for you to be responsible for her children's safety, they cannot to be in your yard. This means at least a phone call before she sends the kids over. Unfortunately, if your neighbor chooses to ignore your request,this does not change your legal liability.
She could direct her children to play in your yard, and turn around and sue you if one of her kids gets hurt.
I, myself, would take it to the level of putting up a fence to protect the neighbor's children and myself.
As far as how to approach the neighbor in a diplomatic way, you might point out some potential dangers associated with unsupervised play. (Kids jump out of swings and break their arms all the time.) Tell your neighbor that you've been thinking lately that your children are getting to that accident prone stage, and that you would be devastated if one of her children got hurt playing in your yard, and the two of you should come up with some new rules for all of your collective children. You'll only come off as snobbish if you suggest that your children are well behaved, and hers are not. If you approach the situation as a problem to be worked on together, you should be o.k.
Let me know how it works out.
Good luck,
D. T