I would wait a couple of days myself. BTW I didn't use that baby pool until the second week, I had rocks on the inside for them to climb out on. When they first went in the water it was a smaller plastic tub, maybe similiar to what you already have in the pen with your ducks.
I let them dry and then crunch them up into little pieces. Then I put them in a dish for the ducks.
If you are using eggs from a store and want to feed them to your ducks I hear it is best to bake them at 350 degrees for a while to kill any germs first.