What feed are you using? If it's chick starter, that doesn't have enough niacin in it for many ducklings, so a supplement is needed, either plain niacin capsules dissolved in water (100 mg per gallon of water) or a sprinkling of brewer's yeast on their food.
I would get a thermometer. 90F the first week, dropping 5 degrees a week, and they need a space in the brooder away from the heat source.
They need water 24/7, as well as starter food. They need to be able to wash their heads frequently, and a bath daily in warm water (90F the first week, similar to the brooder temperature) is a good idea. Water just up to the tops of their legs, and only for a few minutes, and make sure they are warm and dry afterward.