Basically you have to determine what "zones" your kids will be using.
http://www.zvv.ch/preise_zonenplan.asp
Then you can buy a yearly or monthly "Netzpass" (network pass) which allows them to travel certain zones.
http://www.zvv.ch/preise_netzpass_bonuspass_06.asp
"Jugendliche bis 25 Jahre" is the youth fare.
"Jahresabo" = 1 year
"Monatsabo" = 1 month
Hope that helps. You can order the pass online, or buy it at any train station. You will need a passport photo for each kid.
Price table of the ZVV (traffic organisation Zürich)
Ticket validity is either one year or 30 days. First class tickets aren't worth it because of the higher price, short travel time and busses have only one comfort class anyway.
Tickets are either personal or non-personal. Personal yearly tickets can be replaced if they get lost. Non-personal ones can be loaned out to other people.
There are some special prices for personal 2nd class tickets for passengers under 25 years.
People over 16 can get a Halbtax card for lower prices on both ZVV connections (canton Zürich) and SBB/CFF tickets (whole Switzerland). This was covered in this forum in the past.