Intoroducing older ducklings to new ducklings?

We recently got 2 new ducklings who are now 1 week old. Our 3 older ducks are 4 weeks old now and we took the little ducks outside to meet the new ducks the other day. I put the little ducks in the yard close enough they could see each other but far enough that I could interveen if needed. The big ducks went after the little ducks, nipping and trying to stand on them. I stopped them and told them no and after a little bit they sort of gave up but the next day the same situation.

Is there a good way to introduce them or should I just keep letting the little ducks outside while the big ducks are out? When they start to get bigger will the big ducks accept them a little easier? Will they eventually get along? With how warm the weather is, and I refuse to turn the air on, the little ducks will likely be going out with the big ducks in a week or two IF they get along. They are a week old today and they don't even sit under the heat lamp now. I got the heat gun out my husband used for racing and wher they are laying it is 75 degrees and it is 94 under the heat lamp. I don't want to chance them not getting along at all but as of right now the little ducks are to tiny to be in with the big guys if they are not going to be nice.

Any advice on how to get them to get along or accept the babies would be greatly appreciated.

I would separate them but make them visible to each other. Make sure their little beak, etc cant fit through and get bit by the older ducks. I wouldnt trust it until theyve spent time together separated and are more accepting that they are part of the flock now.... And until the babies are big enough to defend themselves a bit and have feathers as protection.