How can i adopt a child from a family that doesn't want it?

i know this family that has two children that the father took in from his (junkie) ex-wife. one is his and the other is Just his step child. he doesn't take care of them as much, but has his mom do all the raising for him. the mother (the children's grandmother) cares for the kids but has health problems and gets tired a lot. i want to take the kids off their hands but what do i need beside written conscent form the parent??