I generally just do what a mama bun would do, and gently press the head down for a few seconds. Seems to work well, especially with babies. Key is to make sure he isn't inadvertently being rewarded when the behavior occurs, such as putting down when he nips while being held, or moving out of the way for him. Will be hard to correct though, when it's kids that it's happening with. Have to teach the bun that he isn't 'boss' of the kids. Will need to spend time supervising interactions, so that if it occurs, are there to help kids safely and gently, correct the behavior, so bun learns to respect kids as well. Gently but firmly is important. Need to be gentle as don't want to scare or hurt bun at all, but need to also be firm enough that he understands that behavior isn't allowed. I would do it with the kids hand under mine, then gently placed on the bunny. This way the bunny does associate the child with the discipline, but I am there to make sure it's done right.
A few cautions though. A rabbit that feels like nipping is ok, can progress to an actual bite, and it may just be best to replace the rabbit if at all possible. Or it may be necessary to not allow the kids to have contact with that bunny and only work on correcting the biting behavior with adults doing it. Then once the behavior seems to be corrected, see how the bun is with the kids.
But as you know, there will always be a risk of a rabbit possibly biting, especially one already inclined to nip people.