You have a wheelbarrow full of crap to deal with.
But let me speak to your presenting question.
How and what to tell the kids regarding rehab.
btdt not so long ago. She got the T-shirt (says Rehab is for Quitters), but I got the hard life lessons on dealing with the kids.
Short and sweet version is -- Open and Honest.
Let them develop and ask questions. They will come up with questions not only now, but for months after as they sort all this through in their minds.
For our oldest I found some Happy Ending Rehab stories on You Tube. That let our daughter (now 11) know that folks make it through and can come out Okay. We also watched some videos on the demise of Amy Winehouse -- about the closest look and feel to Mrs. Hammer as she dropped weight and would zonk out, so our daughter could know the risk.
For all of them -- we (just me and the kids) made up songs. Stuff they could laugh at about all this. We wanted to do a welcome home party. So we re-did Amy Winehouse's "Tried to Make Me Go To Rehab" song as "Welcome Home from Rehab." Did the Addam's Family as "The Addicts Family." We would sing them at the supper table (when the kids would really be missing Mrs. Hammer) and laugh and have joy. Learned that an Army Troop Leader -- that which you can laugh at -- you do not need to fear.
But really, her coming back was harder. We had no real support from the rehab center at all. In fact, they were pretty much total a-holes when I even called to just get basic information. In that regard I would strongly recommend against that particular one. If you are thinking of one in Florida, PM me, and I will tell you the place to avoid.
Mrs. Hammer was whacked out for months when she came back. Really rough time for me and the kids. Daughter wound up in Ala-teen. That was really good.
Oh well, I am rambling. Write back more of what you need to know regarding the kids and rehab, and we will do that deeper.