5 days old is pretty young to already have fleas. Are you sure they are fleas? Do you know how they got them? I suggest their brood area needs to be cleaned and treated with fresh bedding put down.

Diatomaceous earth is not a great treatment for fleas, although can be added, with extreme care, to dust baths. The ducklings, however, are unlikely to be dustbathing. DE can be harmful to lungs if inhaled so if used, handle with care.

Products labeled for poultry are safe for ducks

- Martin's IG Regulator for Flea, Roach, Ant Control - Jeffers (jefferspet.com)

- Prozap DyFly Aerosol Insecticide - Jeffers (jefferspet.com)

From Merck's veterinary manual

The Western chicken flea, or black hen flea, Ceratophyllus niger, seems to be confined to the Pacific coast area of the USA and Canada. This flea breeds in droppings and feeds on birds only occasionally. The European chicken flea, C gallinae, is found worldwide. It breeds in nests and litter and is on the birds only to feed. In addition to chickens, it attacks many other birds, as well as people and domestic pets. Heavy flea infestations cause host emaciation and reduced egg production.

The most important flea control measures are to remove infested litter and to dust the litter surface with carbaryl, coumaphos, malathion, or pyrethroids to kill immature fleas. Insect growth regulators such as methoprene are also effective. Sticktight fleas can be controlled by topical application of insecticides.