To put it bluntly, government advice is that an overnight stay is possible provided you all social distance. I would say that even with the best of intentions, young children can’t/won’t. We have a similar situation but our gc are younger. I think on balance that to put a child in that position, is unfair and also if they were mine, I’d worry about the emotional impact of having to social distance for a long period would have. It is our whole family that want to visit. We have seen them for a day but we have decided that we can’t extend that for longer with the way things stand. If someone became ill, how would we all feel? If we would have regrets (which we would) , then it probably isn’t the best thing to do. We have in reality, no idea of the risk, but it’s important I think, to do what you feel comfortable with.