Hi.
You are welcome.
(At least) 3-months-old, preferably...
Older and bigger they are, easier it will be for Stella to accept them.
You can try once your ducklings are 2-months-old (if you want), but in this case, you need to monitor them carefully AND still keep them separated from Stella at night - just in case, because even if your girl appears frienly, you can't be certain she won't kill them once you are not watching over them...
If they are like the actual French Duclair ducks, then it is a breed that grows fast.
At 3-months-old, my (French) Duclair were heavier than my 3-months-old Saxony drakelets!!
"At the park"?
...Are they abandoned ducks?
You have chosen a good breed!!
In my experience : in addition to being beautiful ducks, Duclair are really decent layers, and let you approach them even if they have not imprinted on you!