In the movie "What Happened to Monday" the grandpa was able to tweak and get 1 (original) and 6 (knockoff) bracelets for each of the sisters.
If he could do that why couldn't he just ask each of the sisters to alter their appearances a bit, have them leave at different times (apartment monitoring) and have them live their lives everyday instead of one day a week? They would still be registered as single child due to the tweaking grandpa did right?