chubby is the same as fat but is more usual when applied to children
It's more usual to say they have chubby cheeks.
"Babies often have chubby cheeks." is a sentence one might hear. It would evoke images of a cute, wholesome, smiling child. To call a baby's cheeks "fat" would be considered very rude and offensive.
To end up with the usual English description of "chubby cheeks" I think the French might be "Les bébés ont souvent les joues potelées". - It would be useful to know what is usual in France!
Its brown fat it helps them keep warm