@_neekko_ My girl nips occasionally, deliberately, but without malice... in the same way as she's scratching an itch and gets too close to the skin. Sometimes it bruises - and it would be really alarming for a child to go through that. I just say "ouch!" and rub the spot. She's done this 3 or 4 times. It's been quite a while since the last time. It would probably be difficult for a young child to be able to control their reaction, but if you can teach her by reacting (calmly and without much ado) that it's not fun for you... maybe it would help train her to stop. Maybe?
That said, I get that you think the nips are a reaction to outside stimuli... my only advise on that is for you to practice walking her (alone) with confidence and control. Focus on acheiving successful solo walks. If nothing else, it will give you a chance to observe your dogs reactions. Maybe your dog is stressed out because she perceives danger for your children and she feels like she has to protect them. (If the kids aren't there, is she still acting the same way?) Maybe your dog is anxious because of bad interactions with other dogs. (Is she still having issues even when it's just you and her?) Give her a lot of positive reinforcement (treats and praise) each time she passes a dog without a negative reaction.
Last idea: help your girls mend any anxiety they may have regarding the nips... smear a dab of peanut butter on the palm of their hands and offer the treat (one at a time). While your dog is slobbering all over your girls, get them to tell her what a good girl she is and give her lots of belly rubs after the gigglefest subsides.
I hope we get to hear more about what you tried and how it worked.