I think that kind of information is best shared face to face in a conversation. The R with your W is so emotionally charged right now that she's not going to hear that type of info coming from you without all the baggage getting in the way.
And I would guess (mindreading here) that she knows the kids are having trouble with this, that's why she reacted so defensively.
Maybe next time suggest that she call the teacher so she can hear it first-hand from a person who isn't emotionally involved.
Her R with the kids is up to her but don't do anything to get in the way of that R. You continue being the best Dad you can be, supporting the kids, supporting their love for their mother.
There's an insightful article here about kids and D
