A doctor is a person who is helping little children.
(picture: A female doctor is examining a little girl.)
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If a kid describes the picture like that, is it grammatical based on the definition of 'doctor'?
it would be better to say, "A doctor is a person who helps little children."
Based on the definition of "doctor," the sentence you provided is not grammatically accurate. Although there can be doctors who specialize in treating children, the general definition of a doctor includes someone who is trained to prevent, diagnose, and treat illnesses and injuries in people of all ages, not just children.
To make the sentence grammatically accurate while still emphasizing the doctor's work with children, you could say, "A doctor is a person who helps people of all ages, including little children." This way, you acknowledge the broader scope of a doctor's practice while highlighting their assistance specifically to children.