Yab-slightly-unreasonable. It was fine for him to go before with 2 DC at home, a full time and part time job. The biggest issue appears to be the school run, which was always going to be an issue that week. So I agree with asking school if you can pick one up earlier each day as you're doing both.
With regard to helping your daughter at home, get their sibling to help with fetching and carrying as much as possible. Or add a bag to the zimmer frame so she can fetch and carry her own things. Use water bottles for drinks instead of open cups.
My sister was in double full leg casts a lot when we were similarly aged and she found ingenious ways of getting around. And what she couldn't do, I was expected to help her with when it came to playing and entertaining ourselves.
That said, if there are additional appointments that week because of your daughter's injury, this is where your husband is being unreasonable to still go on his jolly. Women are nearly always the ones to give up their work to attend the kids appointments. As your dh has a week off work, he could easily help out with the additional appointments and potentially half days for dd at school as a full day in a full leg cast, presuming he is due to go imminently and she hasn't been in a cast more than a few days, is going to really wipe her energy to start with.
But, I would take a long, hard look at the dynamic of your relationship because looking after 2 kids and having 2 jobs inc 1 full time isn't sustainable unless you're planning on leaving him and the side job is allowing you to save for that.