If they are still married AND there are NO court orders (this includes child support) with either Oregon or Arizona AND he has lived in Arizona for more than 90 days, then he could legally not return the children and file for divorce in Arizona.
That's the "good" news...now for the "bad".
You're first going to have to deal with the fact that the "status quo" that has been ongoing for several years now that is that the children have lived primarily with their mother. This also creates a "jurisdiction" problem since Oregon will most likely want to have said jurisdiction over the children and Oregon is one of the MOST father unfriendly states with the exception of a few counties (Marion County is favorable).
You're going to have to come up with reasons why the "status quo" should be changed and any allegations you make will need to be backed by evidence/witnesses. Doctor's reports, school reports, testimony by neighbors, police reports etc.
And this issue with "foster care"...why didn't Dad go for custody back then? What is so bad/different NOW as opposed to THEN?
If you air your dirty linen in public, expect people to comment on the skid marks!