My DD is only 30 months. I left her dad when she was 15 months. I plan on using the line "Your father was/is very ill and because of that, it wasn't safe for him to stay with us". I expect there to be more questions as she grows up, and I fully expect to be the Bad Guy for a time, while she figures out who is responsible and how she feels about it. I hope that eventually she and I can discuss things, but I'm trying very hard to let go of the outcome of it all
Obviously, this is all in the future...I'll let you guys know how it goes, in...5 years or so.