How old are they and what breed? many domestic ducks are too heavy to fly, Pekins and Rouens are 2 that can't. Some of the other domestics can even when you read they cannot.
at 7 weeks they are still growing in their feathers so no worries yet. Most likely the Rouens won't fly the other 2 breeds may try but I haven't heard of anyone saying their Buffs or magpies flew off. I have Muscovy so i have to clip one wing to keep them grounded. i remember someone posting about their swedish duck flying off and of course mallards. So don't worry right now since they are still babies. Just watch when they grow in their flight feathers. There is a Buff thread and a Magpie thread on here so you might go there and ask.They are about 7 weeks old. I have 2 rouens, 2 buff, and 2 magpies