Yep, just ask. I agree that you can't expect them to be mind readers. They are family aren't they? I don't know about how your family operates but we are very open with each other. I am comfortable to ask for anything, and they are comfortable to say no.
So yes, I do think that it is a little silly to be upset that nobody has offered to take the kids for a weekend. Nobody has ever offered that to us, but we have asked plenty of times (and they have said yes )
Go on, just ask them. And hopefully they say yes. If they say no, then I think you have every right to be a bit peeved