When we have one showing a leg problem we immediately think of a niacin deficiency and once they have one it best to get them on Durvet liquid B Complex TSC has it it will say injectable but we give it to our ducklings orally since your feeding this one separately you could mix 1/2 ml into a small amount of feed and let him eat it all that way you k ow he got full dose. You might want to keep this little one in the same brooder you have set up but with a divider of some kind so it’s not getting stepped on by the other and they can all still see each other. Until this recovers an get strong enough to with stand the other pushing and shoving. Sounds like you have Pekin and white layers. Both get big so you want to make sure they get an adequate amount of B everyday either through the Durvet liquid b complex or by putting 1 tab of brewers yeast to every cup of feed you give them. All except any who are already showing issues like this on you described. Sounds like you have a house full.And I commend you for teaching your children how to treat these ducklings and care for them.