op is there more to this than you've posted here?
You've already posted that your dh's relationship with his ds has broken down, and now that his dd no longer wants to see him. What are the reasons behind the breakdown of his relationship with his ds and what has he done to try and resolve it?
I'm sorry to say but your dh is being incredibly weak here. It's not enough to say that he doesn't want to upset his dd. Is his relationship with her worth risking for the sake of not upsetting her? because that's what's going to happen if he doesn't pull his head out of his arse and do something.
As his children's mother has died the stepfather has no legal rights. none at all. So your dh needs to go and fetch his dd, either from school or from her sf's house, with police involvement if need be. How old is his ds, if under eighteen then I would say he needs to be brought back as well.
It may be that your dsd wants nothing to do with her dad, but it may also be that her sf has taken her phone and blocked his number so she can't receive texts from him, but she may not realise this.
Your dh needs to act and he needs to act now. The courts don't actually need to be involved at this stage - she is his dd and he has parental responsibility. If the sf wants to go to court then let him. But he has no legal rights over your dh's child.
So the question is, what is your dh going to do about it now and if not, why not?