I think it is fair enough, especially if you live in a flat
Sometimes the council runs free/cheap sessions. A few years ago I sent them to a veey cheap but fun lego-stop-animation movie making course (Southampton I think in a museum? Council run) 3 sessions of 3 hours, it was fun for them, inspired them to do similar movies at home (takes hours) and I got a few hours off to mooch around on my own.
In your shoes, I would not "sell" it to your DH as time off for you, I'd sell it as a fun activity to stop kids from getting bored
Sometimes even b&q run sessions, or the applestore.
Go researching and find fun cheap stuff for them to do, if you end up with a few hours off it is a bonus.
Or a sport they love (cricket)
Make it specific to their interests
And to hell with the sanctimonious idea that you are not just supposed to look after and entertain them 24/7 for 6 weeks, you have to enjoy it 24/7 too....nonsense. They need stuff to do, and hopefully that means you get some time off too 