The older ones push the little ones out of the way a little bit but the babies are persistent and push their way back in. I'm not concerned with the little ones being pushed out of the way because they always have constant access to food and water and it's not like the big ones are eating and...
I let my 3 five week old ducklings meet my 4 two week old ducklings in a kiddie pool that was filled a little. I also put some ripped grass in the pool so all the ducks were too busy eating and playing to worry about each other and when they noticed each other the older ones pecked at the babies...