You have four people in your M. OW, MIL and WH.
The sooner you get away, the better. Meanwhile, begin acting as though you are D. Consult a lawyer (solicitor?) about your rights. Document everything, start gathering your financial info, proof of bills paid and money spent.
It will never be a good time to tell your kids, so please don't sacrifice yourself in an effort to wait. It will set back your healing enormously. As long as you are loving and supportive, providing someone that they can talk to (such as an IC or family counselor) that will be what will mitigate their ability to handle it more than timing. Though I agree that doing it before school is best...my niece found out about her mom's A in August (my SIL abandoned the family) and she had a really tough time concentrating in school.