Wataru Ikusabe was just a normal fourth grader - until he was somehow transported into another world and dubbed its savior. In this world, the evil King Doakudar has taken over Mt Soukai and caused the rainbow in the sky to turn white, and it's up to Wataru to defeat him and return the colors to the rainbow. With the help of the young Shinobu clan leader Himiko, gruff Shibaraku and the feathered spy Kuruma, Wataru will climb the mountain and bring down the forces of evil with his mecha Ryujinmaru!
In the year 2050, a violent earthquake on the moon spontaneously changed the moon’s atmosphere and gravity, making it habitable for humans to live upon. Fifty years later, the young Daichi Haruka heads to the moon for summer vacation; he wants to try out his newly created Jetboard, but he wants even more to catch a glimpse of the rabbit person recently spotted in the desert. After running across a little girl with furry ears and her like-bodied grandmother V-May, Daichi finds himself in the middle of a grand adventure! With the help of the giant mecha Granzort, his new companions and two other children, Daichi will help put a stop to an evil monster race that is dead set on controlling the universe!
Wataru and Granzort are remarkably similar as far as tone and feel; each involves a young boy who is thrust into the cockpit of a mecha and is responsible for taking down the ultimate evil. The characters' chibi-style appearances are also quite similar, as are the attitudes of the main characters. There's pretty much no chance that if you liked one, you wouldn't like the other too.
Naruto Uzumaki is a young ninja who bears a great power hidden inside him, a power that has isolated him from the rest of his village. As such, his only dream is to become the Hokage - the most powerful ninja, and leader of the village; but first he needs to graduate! With his inability to perform even the most basic ninja techniques, it seems that all Naruto has going for him is his determination to succeed no matter what. Teamed up with the genius Sasuke, book-smart Sakura, and their team leader Kakashi, Naruto embarks on his quest to become the Hokage. But with outside forces posing a threat to the entire Hidden Leaf village, Naruto discovers that he must become much stronger if he ever wants to realize his dream and protect the friendships he's forged.
Combined, Donatello, Leonardo, Raphael and Michaelangelo make up the pizza-loving butt-kicking Teenage Mutant Ninja Turtles. With the help of mysterious Mutastones, the gang are able to perform a Super Mutation and transform into the powerful Super Turtles! However, the evil Shredder, Bebop and Rocksteady have their own dark Mutastone which allows them to similarly change into powerful foes. Along with their friend April and the wise Splinter, the Teenage Mutant Ninja Turtles will battle Shredder's gang and Krang, and even a long-dead ninja!
I know this seems like a relatively strange recommendation, so consider it a gut feeling. TMNT and Wataru both have an old school feel and revolve around fighting the various monsters of the day. There are evil forces at work, but none are overtly scary or intimidating. Just see what you think!