In this case I feel speaking with the BF or the parents would have no effect. The next time those kids are in your home, right after they take off their shoes at your door, you should have them sit down and tell them your house rules. Explain to them what they can and cannot do. Maybe even have a few board games or some crafty items or a movie for them.
I would also tell them straight out how what they did to your sewing room made you feel,and you know deep in their pockets they have more respect for the things of others. Make sure you ask them if they have any questions and if they understand, and make sure they all say "yes." Also, if they do not feel the need to follow your rules and for some reason they come back to your house, they will have to sit in ___ chair/step/sofa/porch and not move until they leave.
It may work, it may not, but they did it and they need to be called on it. At least you know you tried.
edit: Another thing you could do although you probably don't want to do anything for them, is sew each up a pillowcase or something small. Wrap them as a gift. After they agree to the rules present them and watch how they react. That should give you a little insight. They're just kids. They can't help who their parents are.