Unfortunately rice isn't nutritionally complete for ducks and lacks important nutrition they need to grow properly. I would remove the rice from the ducks' access.I have told her not to and I have bought duck food but the seem to get in the bags of rice and only eat that
It can be challenging when people don't listen. Maybe explain to your niece that her actions are hurting your baby ducklings. Personally, I would not allow someone near my ducks if they refused to follow my husbandry rules. Good luck, I know it can be a tricky situation with family.