There will be a point where we will need to take the kids money and put it in our mortgage, otherwise they will be taxed on the interest. I am not looking forward to that day and having to explain that one to her
I'm going slightly off topic, but the kids accounts would have to have enough money in them that they are generating over $18,200 a year (each) before they even start to pay tax (as it's treated the same as normal income). In a pretty standard savings account that means they would have to have around $400,000 before they pay anything in tax. If you are worried about paying a significant amount of tax they would have to have a lot more than that in their accounts. If they are lucky enough to have that much then that's fantastic and I'd be giving MIL a pat on the back hehehe.