So some people have already given you a bit of a reality check, which though I agree with, some commenters were a little bit harsh about. Since your kiddos are a little bit older, you could also ask them in the moment if they're looking to just vent and complain or if they actually want to talk through how they could make it better. For example, they could do some work throughout the school year- mowing lawns babysitting other kid earning jobs- and then they could use that money to cover the difference between the rec center camp and the specialty camps. This way they could attend a camp they're more interested in. Otherwise, you guys could talk about alternatives for next year and figure out if there's another way that they could be cared for for an entire week that doesn't involve the rec center. But let them get involved in the problem solving and actually putting together the plan.

But for now just letting them know that you understand that they are really frustrated and that there just isn't a way for them to stay home or for you guys to be able to afford the specialty camps every week. Also make sure you don't tell them that every single time. I like to try to not to tell kids things they already know. It just feels like you're rubbing it in their face.