Tough choices

Nate has been gone for a week now.  Ryden doesn't know the difference, but Harper does.  Most of the time she isn't worried about it.  There are times when she asks about him or just mentions him while she's talking.  I'm just not sure how to deal with it.  How do I explain to her that Daddy needs to get help and I can't be responsible for him anymore?  Hopefully someday they will both understand that this decision is what is best for all of us.