You aren't taking inflation into account with any of this. Today's £11K won't be worth 11K when they are 18.
How is £100 a month £600 a year?
If you want to save and invest, there are many ways to do it that could give a better return. For example, Premium Bonds where there is a chance of winning something which is above the interest rate gain. Just shoving money (which you can't afford) into an ISA is not necessarily the best way.
You clearly need some proper financial advice across ALL your finances.
You talk about giving your kids a house deposit- but what about your own house? You say you don't have a mortgage. Do you own a house outright? or are you renting?
You need to be honest about your motives for these ISAs. It's all about being 'safe' money which you can't touch - because you have a problem with finances. Except now you are- you're withholding their money even if just short term.
You could have your own ISA and hand that over to your kids in time. But the reason you don't is you would be tempted to withdraw it.
I have the feeling your H is a bully, he disapproves of how you spend and you're dipping into the kids' money because you don't want him to know you can't buy food. OR he's unemployed and a complete waste of space and equally bad at budgeting.
I know you don't want to explain but your marriage is at the root of all of this - that's what needs sorting.