Welp. My kids just loss their whole summer

My two boys (17&18) just lost their whole summer and are grounded. Yes I still ground my 18 year old because he lives under my roof and I pay for everything. His car, his phone, internet, etc. I normally don't ground him though but they both really messed up. Me and my hubby are fostering a 14 year old. He has a lot of trauma and fears because a lot of people hurt him. He has been trying not to sleep at night because the shadows and shapes in the dark scare him and he's experiencing sleep paralysis. We got him a lava lamp to help with the dark. Not much we can do about the sleep paralysis. He also has stuffed Elmo that his mom gave him when he was 4 but she passed away 6 years ago. He does take it everywhere because it reminds him of his mom but he usually just puts it in his backpack because he's embarrassed by it too. He really has been trying to fit in with my boys. It's hard because he's younger and they are either adult or almost adult but I've told them to cheat him kindly. Today my boys were going to the skate park and our foster son wanted to go. They said yes and took him there. Not even an hour later my foster son calls my in tears asking me to come get him. While they were there my 17 year old went into our foster son's backpack and took out his Elmo and told him they don't hang out with babies. and both them and their friends started teasing him for needing a night light and still having stuffed animals. My 18 year old took his Elmo and threw it over the fence of the country club at the park. He claims he wasn't trying to throw it over the fence, just pretend he was, but that's besides the point. They are both too fucking old to be messing with a child that way. Also the country club wasn't opened so I have to wait until tomorrow to go get it back so foster son is so upset. I told my boys they are grounded the whole summer and are gonna spend it volunteering. I haven't found a cause for them to volunteer for yet. If any of you guys have causes or information on some than can help these 2 get some empathy plz comment them below!

Edit: The reason my kids don't work is because I made it clear I wanted their main focus to be school. I plan on them volunteering this summer so they don't get paid. You don't get rewarded for such mean behavior.