Annie I'm so sorry that happened to you. My kids are only 5 and 7 and though not little angels by any means, know they mustn't go into my sewing room without my express permission. And they would never show such disrespect in someone else's home either. If they did, they wouldn't sit down for a week at least.
It's awful that these people are in a sense, part of your family. That makes it tougher to deal with. I know my kids have friends that I don't like coming over to play because of their behaviour. I usually only permit it in summer so at least then they're outside!
I hope you have the full support of your BF and that you managed to fix the damage. It makes my blood boil! I know that non-quilters find it hard to understand how much this means to us sometimes, but it's just a question of respect for other ppl's stuff - and respect is something sorely lacking in a lot of ppl (not only kids) these days.
Here's a big HUG!! Hope you feel better. :-)