How your H's behavior is affecting your children must be so, so difficult. And of course she is clinging onto that hope... I read that children can do that for years. My aunt and uncle divorced when their two daughters were in middle/high school, and years later after the H had gotten remarried (same old $hitty story, he left her for his high school GF, that didn't work out (surprise!!) and he ended up marrying someone else down the line) the four of them were together at one of the daughters' college graduations, without the new wife. The girls were so happy and one of them said to her mom at the end of the night,... mom, isn't this nice? Maybe you guys will get back together. And it broke the mom's heart that the kids were still thinking that after all those years and she had to say no, sweetheart, it is never going to happen.
Anyway, it is totally natural that she thinks that and (not being a child pychologist) don't know how to handle it, but my guess is that you don't want to tell her he won't come back or anything like that... you just need to make sure she knows you are there for her 100% and love her more than anything and you know it is hard and are here for her to talk to. Maybe there are resources out there for you, books, etc to be sure you know how to support her the best you can through this?
Also, I feel enraged at your H for doing this to your kids. (Sorry, I do!!) Can he please get his head out of his own selfish a$$ and stop thinking about himself for one sorry second and think about how his behavior is affecting the children? I'm sorry if that isn't helpful... but it just is so infuriating to me that these WSs are so caught up in their own needs/wants that they forget that they are parents and their children are watching every move they're making and internalizing them.
One thought-- can you have the bins already taken out and the bird feeders filled the next time he comes over?
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing