Tennozu Isle Monorail station

Can I buy child Suica card at the ticket machine at Tennozu Isle Monorail station? Or do I buy at the ticket office (what is the office open hours)?

Yes,all Monorail stations sales "Monorail Suica"
but no sales JR-East .ver with penguin design card.
just differrent card design and refunds rule no problem.
http://www.tokyo-monorail.co.jp/english/tickets/suica/index.html

Childlen card needs a id(passport or) for age checking.
If under 6 yd not need a card ask station ticket window staff.
price : 1000Y - 10000Y(include deposit 500Y)

Ticket window placed central ticket gate side only
open before 1st train 5:08am to last train and no closing day/dates.
http://www.tokyo-monorail.co.jp/english/guidance/tennouzuairu/index.ht...

Can I buy child Suica card at the ticket machine at Tennozu Isle?

No.

Or do I buy at the ticket office?

Yes.
An IC transportation card designed for child-rate basic fares is issued only through a manual procedure, with personal data of the holder (such as the date of birth) registered.

Concerning basic fares and additional charges for bus and train rides by March 2015, a child rate applies basically to kids who were born during the 6 years from 2 April 2002 to 1 April 2008.
A child rate applies also to (a) younger kid(s) when he/she uses a space requiring reservation (e.g. a reserved seat), when he/she/they travel(s) alone, or when three or more younger kids are accompanied by one adult.

E-money stored on your IC transportation card can be valid for 10 years after you use the card last.
You can return the card for a refund only to its issuer. A (Child) Monorail Suica Card is issued by Tokyo Monorail, and you cannot return it in a JR station.

If you have a chance to travel to Monorail Hamamatsucho Station, you can get issued a (Child) Suica Card by JR-East in Hamamatsucho Station, and you can return it in a major JR-East station in and around Tokyo.
If you have a chance to take a Tokyo Monorail train to Tenkubashi Station, you can get issued a (Child) PASMO Card in Tenkubashi Station of Keikyu, and you can return it in a major non-JR station in and around Tokyo.

Please check "Basic Use" in this article if you have never used IC transportation card in Japan.
http://www.japan-guide.com/e/e2359_002.html