Rosie, moving house, especially when in a chain, is stressful. I can understand that you felt a bit annoyed by their lack of courtesy. However, do they have any direct contact with you? It could be that they were advised to give their new address for insurance and school registration purpose but were not aware that the sale was to be kept private?
Re. Not telling the children, I can understand your point, however they will have certainly picked on the stress you are under, above all when I read your other post. My child also has autism and we had a social story ready to explain the move, as to get him used to the idea (we were moving to a rented property in the interim). He dealt with it very well as he knew what was coming. Of course, your child is different to mine, you'll know best how he is likely to react.