My 4 month old westie loves kids but we don't have any ourselves, so he gets very excited when we meet them out on our walks. They all want to pet him but he keeps jumping up trying to lick them and all the 'sit' and 'stay' and 'down' commands fall on deaf ears. Normally he's pretty good with these at home.
I try to keep him down by holding his collar but it's like holding a wriggling worm. How can I get him out of this habit or to heed my commands? I ask the kids to crouch down to greet him but he keeps jumping and that can scare the littler ones. I know he wouldn't hurt them but they don't, and I don't want him to think it's ok to do this. Treats to make him heed me don't work when kids are around either.
Thanks for any advice!

We are having the same thing at the moment, I don't know if this is what is recommened but we give the children some treats, and ask them to give Jed a treat when he puts all four paw's on the ground, I think it is starting to sink in, or I might just be kidding myself!!! :D

Try getting some 'tame' kids to come up to you but ignore him until he behaves the way you want.  He'll soon learn that the way to get attention is not to jump up.

I had this with my ESS. I got the kiddies to immediately stand upright, cross their arms and turn their backs on him the minute he jumped up - and only begin to fuss him again once his paws were on the ground.

Good luck :)

I think the standing up and ignore option is a better one than asking children to crouch down because they then make their face into the perfect nipping target for an excited puppy. I have a similar problem with my 16 week old JRT who is awfully cute and waggy tailed (he saves his wicked behaviour for home!) and, as a small dog, is perceived to be just the sort of nice little doggie for people to invite their children to stroke. He absolutely loves people but he absolutely needs to learn to calm down in the company of small ones so I don't encourage children's faces being put too close to his since if anything will put them off a dog it'll be an accidental nip. 

Standing still and saying hello from a greater height or just ignoring over-excitement does work. But be prepared for it to take some time!!!