Give them the Gift of Language
While toys and trinkets are fine gifts for children, consider buying your child a book about learning Japanese. Many of these can come in the form of picture books that teach them simple vocabulary, such as counting in Japanese or how to say their favorite toys in Japanese. If they become interested, this could be cultivated in learning a second language, which will provide far more joy than any single toy can provide.
The easiest books for children to start learning Japanese words come in the form of picture dictionaries. The books will have a single object or many objects in a room with the Japanese and english name next to them. Make sure the Japanese is in romaji and not hiragana/katakana unless they can already read the phonetic alphabets of Japanese.