Six year old on trains

We will be travelling with two children, one aged 6, and one aged three, from Tokyo to Osaka by train. We will get the Japan Rail Pass for us two adults, but do we need to get one for the six year old as well? If not, would he get his own seat? And will the three year old have his own seat as well?
Thanks

It seems you need to buy one for 6year old son(daughter).
See below the page.
http://www.jreast.co.jp/e/ticket/types.html

And JR Pass prices.
http://www.jreast.co.jp/e/eastpass/prices.html

Assume you are doing more than JUST Tokyo to Osaka??If that's your only trip you won't need a JR Pass at all, just regular tickets.

Indeed, we will be making more than the one trip as we will be going from the airport to Tokyo, then Tokyo to Mount Fuji (return), and Kyoto to Osaka (return).

Thanks for your replies.

By the way, what's the nearest station to Mount Fuji?

Most people come to Fujikawaguchiko Station.

The correct station name is Kawaguchiko Station.

oops! Yeah Kawaguchiko Station, and I live here! lol

The original Japanese Version of the fare rules state in more detail :
''こども 6歳〜12歳未満（6歳でも小学校入学前は「幼児」です）''

Child : 6-11 (but 6 year olds who have not entered school yet are regarded as infants).

Pls note that the kids are supposed to travel on your laps if they don't have a ticket, however, this is ususlly only enforeced for 'reserved seats'.

Child : 6-11 (but 6 year olds who have not entered school yet are regarded as infants).

Since school systems (i.e. age and month of entering elementary school) differ internationally, I heard that just the age 6-11 is taken into consideration in case of kids who do not live in Japan.