I'd address this problem both on the inside and the outside! Start giving them all WARM molasses water to drink. This is a chicken laxative, great for chicks. The recipe/ratio is one large drop of food-grade molasses per tablespoon of water. Give them 7-12 sips per day (being careful not to submerge their nostrils if you need to hold them to make them drink, otherwise they will aspirate). Usually giving them molasses water for 3-4 days will remedy the problem. Make this molasses water fresh, daily, so it won't ferment. Also, make sure that all drinking water is warm to tepid (they need regular drinking water in addition to the molasses water, of course), as cool water can upset chick tummies. Don't give the ducks the molasses water. Make sure the chicks are eating medicated chick starter feed. Do not allow the duckings to eat the medicated feed, as it can kill ducklings. In addition to this, externally wash the dried poop off their bellies and vents using warm water on a soft wash cloth, and then dry the chicks thoroughly and place back under the heat lamp to warm them. Only use olive oil on irriated red skin near the vent, else it will cause dirty bedding to stick to the belly and further irriate the skin there.