roebuck220 offers good advice.
Personally I would feed slightly more and I would divide the feed into two offerings - AM and PM. Young ducks have a tendency to wander and two smaller feeds helps to keep them close. Leave them wanting more! Think about how you feed too - especially of you have problems with gulls. I tend to feed densely until the birds are bigger when I spread it more.
If you have suddenly lost a lot of your charges look around nearby and when you find them gently walk them back to the water - it can be done.