I think your first priority is to get help for your DD.
If you feel you can't approach the school for counseling for her, then please trawl through Amazon looking for books that explain that families come in all different shapes and sizes and that one with 'just' a mum is a perfectly fine family, and also books dealing with grief and abandonment.
You won't find many on abandonment, sad to say. It's an issue that affects hundreds of thousands of children and there is precious little written for them that they can approach on their own level.
So I would try really hard to put any friction between you and the school behind you and pester them for counseling for your DD and actually the rest of your children too. (Though you are naturally preoccupied with your poor DD and her quest for an alternative dad, your other DCs are also feeling grief and pain at the loss they have suffered.)
Meanwhile, try to put forth the massive effort to do things as a family. A cheesy family game night, a family walk once a week, a family movie night with popcorn on the couch, family photos stuck on a noticeboard that is displayed prominently, and notes to the children from you expressing affection, appreciation, and encouragement might all go down well and reinforce the idea that you are a viable and solid family regardless of the shit that has been dumped on you all from a great height.
Is there any way your father could make a visit every once in a while? Could you visit him? Or could he and your DD facetime or skype, or somehow talk to each other and develop an affectionate relationship?
I would tell the children when they are young teens (12/13ish) if they are all close in age.
If they're not close in age then you are looking at older ones keeping quite a big secret until the younger ones catch up.
What is the age spread?
When you talk to the children about their father leaving, make sure you emphasise over and over again that this was nothing to do with them, that you and he split up only for reasons to do with the relationship between the two of you, not anything to do with the children, and that his choice not to have anything more to do with the family is down to a character flaw in him, again not at all related to any of them. This is so hard for children but you have to be very explicit.
Make sure they all know you are there to chat and that no feeling of theirs about their father is unacceptable. They may have great fondness for them despite all. This will be hard for you.