The watch is less suitable for a child's wrist as it is very elongated. As far as the functions are concerned, it depends on what he wants or needs to use. For me, it's a sleep and movement detector and the data is collated in an app on the smartphone. In my opinion, smartphones don't belong in children's mobile phones.....

Fitbit Inspire 3, 2 different lengths of wristband are supplied

The Fitbit is less of a watch and more of a fitness tracker. The main functions are the recording of activities (heart rate, distance, time, etc.) during sporting activities (running, cycling, swimming, hiking, etc.). The fact that the time is also displayed is more of a secondary function. The data is stored and analysed in the smartphone app.
Theoretically, the Fitbit can be used purely as a watch to display the time or as an alarm clock etc.. However, it is certainly not designed for this purpose.

I tested the watch on my son, who is also 10, and it fitted. The functions can also be used without a smartphone, as it is not actually a smartwatch.

The display is reset to zero at midnight. The history is only visible via the app.
However, daily data is visible.
Pedometers, floors, kilometres etc. could be updated manually every day.
The data is not lost, so if he later has a smartphone, the data can be transferred there.