Goodluck with it, but personally I would be holding off a final decision until you see them.
I totally understand the decision you've made now, and honestly I would have made that decision if I were in the same situation but I would have held off making it the final decision until we got there and could evaluate the situation properly for ourselves. there are so many factors besides the scabies, that could weigh into whether or not they hold the baby. Dh's grandparents had dementia, sometimes when we would visit they would be in the right headpsace to have cuddles, othertimes we really thought it better that they didnt. Perhaps they werent in the right head space, or they were having a rough day, tired and what not.. And that wasnt to mention the general mood fo the girls, some days (especially when they were in hospital, and even now when we visit in the nursing home) the girls wont go to them. They just scream whilst being held.

Goodluck and i hope it all goes well, but maybe dont be afraid to change your mind if it doesnt sit right with you there and then?