Let's go a bit old-school.
Manga:
Mazinger Z
Getter Robo
Astro Boy
Doraemon
(You never really specified the era and the age preference, so there's that... :P)
Anime:
Genesis of Aquarion
Bokurano
Doraemon
(Yeah, "Doraemon" ain't really a "mech" anime, but the protagonist's a robot. It's also...