Yes, I agree that it’s ok to put them into a secure outdoor area during the day when it is warm at this age.
“Pool time” is definitely in need of supervision, but as long as you are confident that they can’t escape the day pen, and you have the top covered securely, IMO they are good to be out inside the pen even if you need to leave for a bit. Just leave them with food and a way to get a drink but not a “pool”
My thoughts / suggestions may change if you live rurally and have small predators (weasels, lesser weasels, snakes large enough to take ducklings that size...)
We live rurally and have coyotes, many predatory birds, raccoons, the very rare ermine, bobcats, and cougar here. No “bad” snakes, but the “bad” snakes are the most common daytime predator in many areas.
I also use a collapsible “dog” fence for my day pen for my babies. I have 1/2” hardware cloth zip tied to each of the folding panels, so the babies can’t squeeze through, and the likely predators can’t reach through.
I also have some 1”x4” fencing that is bent to shape that I can place over the top to make sure the turkey poults stay in, and keep birds out. When I move the pen to a new/ clean area, I just keep it the same shape so the “top” can go back on.
In my experience, the more exposure the ducks have to a bit of swim time, and time on safe outside ground/ grass for most of the birds we keep, the more healthy and hearty they tend to be as they grow.
Just my 0.02$