I don't remember much about the cartoon or anime but there is a scene in which thunder falls on a small robot and from then (the hero is a boy) it's like the hero controls the robot and fights the evil. When I think of it I remember the Big Hero 6 robot in a smaller size. It was nearly 5 to 9 years I guess or in the middle of 5 to 9 years.
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1Welcome to Science Fiction & Fantasy! These might help you to improve your question; How to ask a good story-ID question? and How to ask a good (anime) Story-ID question? – Valorum Sep 18 '19 at 15:40
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Does the robot grow to a larger size or does it stay small? – FuzzyBoots Sep 18 '19 at 15:57
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Do you mean thunder or lightning? – Reinstate Monica - Goodbye SE Sep 18 '19 at 17:08
I think this is Heroman as per my answer here:
Orphaned American boy Joey lives with his grandmother, working at a restaurant in the West Coast city of Center City (センターシティ Sentā Shiti, based on Los Angeles), while going to school. Upon hearing of a new toy robot called the Heybo, Joey believes that getting one will make his life better, turning him into a hero to protect his friends and family. However, he cannot afford to buy it on his meager salary. His luck changes when he picks up a broken down Heybo abandoned by a school bully. He tries to fix it, naming it Heroman, but does not have any luck in getting it to work. However, when it gets struck by a bolt of strange lightning, it transforms into a giant robot, just in time to save his friend Lina from impending danger. Now, Joey and Heroman are Earth's only defense against the evil insectoid Skrugg (スクラッグ Sukuraggu) aliens, unknowingly summoned to Earth by Joey's science teacher.
Cover of first volume of Heroman manga
Trailer
Originally found by searching for anime poor kid giant toy robot, and remembered by me as a case involving an anime with a toy robot turned defender.
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1If this is the correct answer, you can accept it by clicking on the checkmark by the voting buttons. – FuzzyBoots Sep 18 '19 at 16:01