This is a crappy situation to be in. I just had a google and after reading some of the info, I don't know if I would want to risk it TBH. I know it is a really really hard situation for you to be in, but scabies are extremely contagious and I'd be weary about taking older kids let alone a baby. From what I read they are really itchy and an infestation can last for 4-6 weeks and the treatment is pretty full on with bubs having to be covered completely in ointments. If you absolutely had to let them hold him, I would be bathing him immediately afterwards and changing all of his clothes. I'd also be worried about them kissing him on the face - because you know all that delicious baby goodness is also just begging to be kissed kwim?