The reason she's doing it is because she hasn't been taught how to act around small children. Small children have fast jerky movements, high pitched voices and are generally always excited. Poppy gets excited and defaults to instinct - to nip and herd.
First I recommend removing her when the kids are really excited, when there's a lot of squealing and running around. It will be too hard to get her focus with that much going on and prevention is the best cure. You don't need to put her in time out if she hasn't misbehaved, snap on a lead or put her in a gated area where she can watch but not run after them. If she getting to silly there's nothing wrong with a time out. Just remember with a dog a time out isn't to get them to think about what they've done, dog's don't work like that, it's to
let calm them down, so let her out when she's calm and quiet.
Honestly, I would not let her around visiting small children unless the behaviour is under control. You can't really train other people's children, and you also don't want her to accidently get skin or have a child tell their parent your dog bit them.
With your child/ren teach 'be like a tree'. When Poppy jumps or nips the child stands very still and crosses their arms across their chest. I know it's not exactly like a tree, unless the branches are gnarled and crossed, but it's the recommended method for teaching pre-school children to deal with a jumping dog.
Also sounds like she needs to learn 4-on-the-floor, so that means no jumping allowed at all, with anyone - that means adults too.
Redirect. Keep a lot of squeaky toys handy and shove one in her mouth every time she gets excited, with or without kids around. As Brittany and Cara said -you want your dog to put something else in their mouth when they feel the need to nip and this way you're teaching her excitement=toy in mouth. I did the same thing for my male. Throw a toy in with her when you've separated her from the kids and when you introduce her back with the kids around make sure there's always a squeaky toy around.
