I joined my group of younger ducks last year to the adult group (their parents and other adults) but it took awhile. I put up a temporary wall of chicken wire in their pen and gave them access to the big doghouse at the end of the pen. They all could see each other and I left it like that for a few weeks if I remember correctly. When I took that wall down, I left part of it up so they could run behind it if necessary. I stayed out in the pen too and watched a lot. The big ones chased and scared the young ones, but overall, it wasn't too bad. 1 drake was a bully, so we would intervene then. We even grabbed him and held him tightly and scolded him. Eventually, they settled in. They are a big happy duck family now, interbreeding, and generally very happy together. This was all when the younger ones were about 1/2-3/4 the size of the adults. Not as babies.
Good Luck!