They can if you want them to and if you can afford it easily, but it's not necessary. Apple Watch has many features for which a child below 15 years has limited use. If they cannot fully use the features, it can feel like a waste of money, especially when the Apple Watch can be expensive. However, if you really want to get them a smartwatch, various other brands are pretty affordable. Maybe you can upgrade to an Apple Watch later on when they're a bit grown, depending on their usage. Plus, they won't be able to use their smartwatch in school because they aren't allowed to be used anymore unless for instructional purposes.