If a hen hatches ducklings on her on, I would leave them with her. The only reason I take away babies from a hen is if she is not a good mother - such as pecking at them, not squatting to let them snuggle under her, etc.
Yes, Muscovies are great mothers, but sometimes we have a duckling-eating Egret show up...it's awful seeing it carry babies away, so we intervene and put the babies in a pen with warmth, chick starter and water - mom too, if we can catch her.