At the age they are unless your offering chick grit what you can feed them to get the liquid b into her is limited. Some ducklings don't mind the taste and will gobble the peas right up. But unless your offering chick grit I wouldn't even give them peas. They have no way to process anything other than their starter feed, Be sure the niacin your giving them isn't time released or flush free.
Lets see putting it into their waterer would be fine but that means you don't know how much she is getting.
Hows the feet doing?
If you can get some chick grit you could crush up some mealworms and put Liquid b on them. Let her get a taste for the mealworms first so she sees how yummy they are. When offering chick grit just put it into a small bowl close to their feed so they use it as they need it.
Lets see putting it into their waterer would be fine but that means you don't know how much she is getting.
Hows the feet doing?
If you can get some chick grit you could crush up some mealworms and put Liquid b on them. Let her get a taste for the mealworms first so she sees how yummy they are. When offering chick grit just put it into a small bowl close to their feed so they use it as they need it.