IMO even bantam sized ducks are going to outgrow that set up pretty quickly. 10 square feet per bird at a bare minimum. They will produce their weight in droppings in a few days. My ducks rotate between a 120 square foot pen and a "duck tractor." That is 6 mallards. They are smelly, needful, loud, always hungry and really messy. I still dig them. I have 14 right now in the main pen and it looks like a bomb went off. It has been raining heavy and they still make a mess.
Be careful with young ducks and water. In the wild, momma puts her oil from her glands on the ducklings. They won't have their own oil for 3-5 weeks, until feathering is underway. They can easily drown, and will chill to death if the temperature drops at night. My mallards don't see a pan or pool of water for 6 weeks, even in summer.