Train travel – should i book/reserve seat for 2 year old son

Hi everybody,

On the ticket booking sites it states that children under (usually) 6 travel for free. I want a seat for my son and are not sure if they get a seat (for free)? Or do I need to book/buy a seat?

For example the shinkansen to Kyoto I booked him as a child so I could reserve a seat.

Does anybody know how this works?

Many thanks.