There's an element of your husband trying to have his cake and eat it here. He doesn't want to take the financial hit of childcare, but doesn't want to take time off. He needs to grow up a bit. The only time you should really work with a child at home is in an emergency situation, not routinely asin the holidays.
You need a frank chat with your husband and your employers about your circumstances and how you can work flexibly to accommodate your children's needs. He will probably say his employer won't accommodate, but they have to by law I think. So sort your half of the holidays out, he can then sort his with his employer. If he doesn't want to take his leave in school holidays he can look after them whilst working instead. You have to force him to take responsibility so this will involve him doing a crap job looking after them and leaving him to it, but it is the only way he'll learn. You may need to go into the office for your half where you are working. I'd just lie to him and say you are getting into trouble, so it needs to be sorted.
You could also work longer days or different shifts to get more hours leave in the holidays, or just have a contract where you have a few extra unpaid weeks each a year.
How severe are your children's needs? Are you claiming DLA if your children's needs meet the threshold? That might cover some of the additional costs if SEN childcare costs you more than a non SEND child. Your councils local offer webpage will show childcare options for SEND places. I do understand as my son's needs are complex (as in severe LD) and there is an expectation that a parent in the household isn't working with SEN kids. You just don't have childcare options like a lot of the posters on this thread seem to think. Most holiday clubs are for very high functioning SEN, not kids who have behaviour issues or may need 1 to 1.
Ignore the hysteria on here and make a plan. You've been forced into what you are doing by stealth, but you need to put your foot down now.
I will put my guess in now that your husband's 'see what I can do' will amount to nothing.