I would say, at the very beginning it looked a bit childish, the story and the art, but for me it changed when we got to know the background of Nami, and how Arlong was opressing those villagers for money, and how she had to survive, from that moment I haven't consider OP childish, sure it has some moments where Luffy, Usopp and Chopper get together and become complete children, but it doesn't have much effect on the story. The only arc where I felt it was more of a fun and childish one was the Dave Back arc.