I would honestly agree with this unless the parent is going to assume care for the animal. By 2 weeks the duck is more than double it's original size, not a fan of being held, a messy mess of water and by 3 weeks often too big for the brooder and ready to hit the coop. It's also a hit or miss if you try to toss them in with the chickens but likely the chickens will peck them. So they would need their own duck house. A nice secure one that has a floor raccoon and dog can't tunnel under, walls that predators can't tear open or chew through...there are a lot of considerations for having ducks. I would research some more before making the final choice. Otherwise mom just got herself some new pets she has to care for, for the next 10 years. <3Woof, I would NOT suggest a duck as a pet for a little kid. They're hard to handle and not very sociable and they grow fast and you're looking at a high-maintinence animal... Which falls to the parents to care for when the kid is no longer interested.
That's how animals end up in rescues or released into the wild. I worked at a pet store and every parent wanted to get a guinea pig for the first pet not knowing that's almost a decade of keeping an animal that bites, scratches and runs away form you at every turn, can't be picked up and isn't nice to pet (wiry fur). I always had to correct them that it was a terrible choice... And the sheer number of animals we had returned to us on a daily basis attests to people who had no idea what they were in for...
Rabbits can be a nice choice for a pet... The dwarf kinds get to be 5lbs tops, they're quiet, low(er) maintinence and friendly. My lops love to be snuggled. They can be taught tricks and litter trained. But I wouldn't get a duck for someone who is under 12.