Hmmm ... it kind of depends on the child. With mine, I would let him know as soon as possible, starting off with something like, "did you know Charlie is going to move to Wonderland eventually? I am so excited for him, because once he moves he'll be able to item A that he has always wanted. But I'm also glad we've get to spend more time with Charlie before he moves, because I really will miss him."
Lol, maybe not the best script, but the point is people moving in and out of our lives is simply a part of living, and we make of it what we choose to. After my children's nanny moved out of state, we made plans to fly out there and visit her; it was a place I had never been, and it helped ease the loss. She showed them on the map where she was going, and she emailed pictures right after she arrived; I think they liked being able to picture where she was. We never hid the pending move, and we never glossed over it. Changes come with good and bad in them, and it all just "is."
_________________
Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).