I LOVE these, especially because Oda drew them, and they could rise a few questions. My personal favorite is Mihawk. He is one of the most mysterious characters, and it's weird seeing him as a kid, and his design could easily be like a manga's protagonist. It looks like he has been working hard since he was a child. Now I really want to know more about him.
An there are 3 other characters which I find interesting:
Crocodile. Looks kinda feminine. There is a crazy theory about him being originally a female, but Ivankov "fixed" him. Somehow I can see this being true.
Kuma also looks interesting. He seems like a normal kid. Kinda makes me curious about his past. I wonder if we ever get to explore it.
Teach as a crying kid is kinda fitting. Must have had a shitty childhood.