
This is a common thing for pups to do as they're starting to get a bit more confident and growing up - I bet your kids have said "No, I don't want to!" when you've asked them to do something, haven't they? It's just the same. But as you are aware, you don't want the pup to go any further than grumbling about it.
When he does grumble, just firmly say "No" and calmly put him in his kennel. If you are trying to take something from him, offer him a 'swop' of a nice titbit when you say "Give", then you can praise him, which will make it easier next time. And I expect he's getting a bit too big and heavy now to be picked up by children - he probably feels very unsafe and afraid he's going to fall - perhaps their fingers dig in and hurt him. So I'd stop them doing that right away.
Have you got a copy of "The Perfect Puppy" by Gwen Bailey? It would give you a lot of help and insight into how you can prevent/stop many problems.
Hope this helps.
:)