The total railroad cost depends on how the little child behaves..., no, rather takes a seat on the train. As to JR trains, if a little child solely uses a reserved ordinary-class seat, a fare and a charge (both of kids' rate) are required.
If a little child uses a non-reservation seat or the same one seat reserved for the adult,
one adult can accompany one or two such children without additional fares or charges.
(Kids' rate is half of adults' rate, with fraction 5 yen rounded off).
I sometimes take a non-reservation seat of Nozomi because its charge price is the same as that of Hikari while Nozomi is the fastest.
But.... Considering the little child, I guess reserved seats will be the better choice.
A JR non-reservation seat ticket is valid on one unspecified train.
As to tickets sold directly by JR,
you can switch from a non-reservation seat to a reserved seat for once with charge adjustment, but not vice versa.
In the article linked from ts's comment, "fare" seems to include "charge."
"Puratto Kodama Economy Plan" mentioned in the article is a package tour offered by JR Tokai Tours,
in which a passenger should take one of specified Kodama trains and pass through specified gates at stations.
I'm not sure whether the conditions concerning the tour are explained in English.
: travel from Kyoto to Tokyo one-way on 25/09/2009
JR seat reservation for that day can be made from 10:00 of 25 August (by Japan Standard Time, UTC + 9 hours).
As to year 2009, that day falls on a Friday between a long row of a Saturday and holidays (19 through 23) and another Saturday (26);
I guess it will be not seldom the case with peoples in Japan to have a mini vacation on 19 through 27 September;
maybe some popular attractions in Kyoto or Tokyo will gather throngs of spectators even on that Friday.
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