In theory, if combined with daily exercise, I don’t see a problem. In practice I dare say it’d be very difficult to stick strictly to the distance, plus it’s against the spirit of the rules, and there might well be Stasi-like, curtain-twitching neighbours kicking up a stink, especially if it was a regular thing.
A Swedish friend of mine (in Stockholm) is visiting her son’s house maybe once a week for coffee and a chat, but they sit well apart on a balcony that I know runs the whole width of the house. However she doesn’t see the grandchildren, the elder 2 of whom are still going to school.
Personally I’d find it extremely hard to see grandchildren at a distance and not be able to give them a cuddle, but then mine are very small so they’d find it very hard, not to mention bewildering, too. I suppose there’s some advantage to living an hour and a half’s drive away, so there’s no temptation.
It must be sorely tempting if you’re just a walk away.