One of the dictionary definitions for imp is:
2 b : a mischievous child : urchin
Not a lot is known about the reproductive methods of Goblins/Orcs, but one can probably assume they are born young.
A year later: Re-reading The Hobbit, I came across a reinforcing quote. Later, just after Gollum discovers the ring is missing, he tries to figure out when he might have lost it. (Emphasis mine.)
"My birthday-present! Curse it! How did we lose it, my precious? Yes, that's it. When we came this way last, when we twisted that nassty young squeaker. That's it. Curse it! It slipped from us, after all these ages and ages! It's gone, gollum."