I wondered about Meliodas' true form for a while and I started suspecting he really is a demon when he first showed his other side.
Why a demon? Well, it just striked me. Considering neither Ban nor Diane were normal humans and I thought maybe none of the sins were human. And the first non-human species that came to my mind and fit Meliodas was a demon. After that first thought I always wondered about it and until now there was nothing against it.
First, demons exist. Doesn't mean anything for now, but at least they exist, which makes this at least possible.
Then, there is his age. In chapter 39 Cain said that Danafor was destroyed 16 years ago and he hasn't aged at all in those 16 years, it seems. He still looks like a kid, so I think it's just a shell. It's not his true form but something that is maybe produced by his broken sword that he doesn't let go of and thus his outer appearance doesn't change.
His injuries also heal quite fast and he couldn't be poisoned. That may also be because it's not his true body but something more or less artificial.
Veronica told Elizabeth that Meliodas is a monster in chapter 28 and that he destroyed the Danafor kingdom (well, he said that's not true, but maybe he just means that it wasn't him but his other side or something? Maybe he lost control?). Either way, a power like that isn't human either.
After Meliodas rescued Elizabeth we see his shadow on a huge rock and it definitely doesn't look human, but more like a demon.
Now, he doesn't behave like a demon, does he? But he certainly is kind of scary sometimes, whenever he is angry. And he doesn't show much real emotion except that. He always looks like he's putting up a front, which might be because he's not a human to begin with and I don't think demons have much emotions either... He could just have copied what he saw until now, so he can look more humanlike.
Now, why is he in that childform? I guess it's some kind of seal. Maybe it was Liz, I don't know. But if it is a seal, it seems like Meliodas himself doesn't want it to break. After all he never lets go of his broken sword, which is probably the seal (whenever he loses his sword, his other side comes out if I remember correctly). This could also be because of Liz, who was really important to him and whose beliefs he inherited after all.
Just some thoughts of mine I wanted to share...
So, what do you think? Any other ideas?
PS: My english might not be right sometimes, cause it's not my native language. Sry for that