OP, I hate confrontation just like you do. I tend to dread those conversations too.
However if you are 100% sure you want to leave this current position, then you must give notice as soon as you know so she can find someone soon.
You are right. She may find someone immediately and kick you to the curb. Make sure you have enough financially to live off of before you give notice.
It's better for everyone involved if you do not stay if you will not be happy. Because if you stay, you will just become more and more bitter and since there are precious children involved here, they unfortunately will be the ones who will suffer the most.
Sadly you may never see them again, but that is the risk you will have to take. I am sorry that may happen. Unfortunately some parents tend to put their own needs/wants ahead of what is best for their own families.
