I'm planning to purchase carnets of tickets for the Paris metro for my kids aged 5 and 11. I wanted to know how the reduced pricing works on both buses and the metro. Thanks!
Other than price, they'll work the same as for they do for adults. Your 11 year-old doesn't qualify for reduced pricing but your 5 year-old does.
Your carnets will cost 14,50 euro per traveler age 10 and over, and 7,25 euro for children age 4 to age 9.
http://www.ratp.fr/en/ratp/r_61656/t-ticket/
Editing to add: no need to purchase carnets in advance (they'll cost more) but the pre-ordering page linked to the RATP website explains age divisions here:
http://booking.parisinfo.com/il4-offer_i168-book-of-10-t-tickets-ratp.aspx
Looks like You can get an "Enfant" carnet for the 5 yo for 7.25 EUR, but 11 requires an adult. However, the Visite pass is half price for children under 12.
How long will you be in Paris and what dates. This determines the value of different passes. And as noted a child 10 and older uses adult tickets on transport. The Visite for kids might be a good deal for the 11 year old. Also on weekends, the kids can get half price day tickets called Tickets Jeune. They are like a Mobilis but half the price. The ticket jeune is for anyone 25 and under on weekends.