Even though it seems I've joined in on this topic a little late and I haven't done more than skimmed through the first page of replies, let me offer up my opinion.
I think spike is still a "child" because even with the mental stability of the ponies hes around, in the episode "Dragon Quest" he was so quick to jump along with the dragon migration after the Rainbow Dash insulted him even with Rarity's coaxing that he didn't need to go. Specifically after Rainbow Dash's saying he isn't like other dragons he stormed off then proceeded to do research about dragons late into the night. The next morning he leaves to join the dragon migration. To be simple I think he's a child because he doesn't really know what he is. All the ponies aren't children because they know what they are even with a few aspects of who they are clouded. Spike though not so much.
I won't deny that Spike has a more mature personality that you see from Twilight Sparkle, but he's been with her since birth so of course she would have rubbed off on him at least a tiny bit.
I wouldn't say hes a child or baby, but he's somewhere in the adolescence area, with the personality traits he's gotten from the ponies, and other traits he himself has developed.