the second Child is 12 but he is still at primary school. So as rules are for Suica Child card he can still use this card. The eligibility depends simply on passengersf age, and the age standard is based on the Japanese school age; whether a foreign passenger belongs to a school in his/her homeland is not relevant.
When will you do this journey?
In April 2017 or later, he who is now 12 will not be eligible for child IC transportation cards.
If his legal birthdate is the 1st of April 2004 or earlier, he is now already too old.
If his legal birthdate is during the one year from the 2nd of April 2004, he should be eligible until March 2017 will end.
Can we use Suica for charge Kansai One Pass (as a special kind of ICOCA card for foreign Tourists)? Well, I am not sure what you are asking about, but a Suica card does not bring you benefits which are connected to a Kansai One Pass.
Our itinerary is Tokyo-Hakone-Nagoya-Takayama-Gero-Hiroshima-Osaka/Kyoto. We will get 7 days JR pass for places between Tokyo and Osaka. In Shinagawa Station,
you can return Suica cards to JR-East for a refund at the JR Ticket Office (near the north gate)
then go into the Shinkansen zone through the Shinkansen (North) Transfer Gate with your Japan Rail Pass.
(JR Shinagawa Station is much simpler than JR Tokyo Station.)
http://www.jreast.co.jp/e/stations/e788.htmlIn Shin-Osaka or Kyoto Station,
you can go out of the Shinkansen zone through the Shinkansen Transfer Gate with your Japan Rail Pass
then get Kansai One Passes issued by JR-West at the JR Ticket Office.
In Shinagawa, Kyoto and Shin-Osaka Stations, the Shinkansen zones are managed by JR-Central, which is the operator of Tokaido Shinkansen (Tokyo -- Shinagawa -- Kyoto -- Shin-Osaka); there you can neither make a refund of Suica Cards nor acquire Kansai One Passes.
We will keep our Cards like souvenirs or for another time E-money stored on a Suica or ICOCA card can be valid for ten years after the card is used last.
I checked here http://www.japan-guide.com/e/e2359_003.html This article seems to have become too long and completed for beginners (due to the historical backgrounds of IC transportation cards in Japan).
The very basic is that you should use the same IC card both when you enter one usage area on the rail network and when you exit it.