Hello Kitty Online!

http://www.hellokittyonline.com

or maybe it was Hello Kitty Island Adventure?

no it's the Hello Kitty MMORPG! and it's FREE TO PLAY!!
for kids of all ages

Hello Kitty Online (HKO) is the first and only official massively multiplayer online game (MMORPG) of Hello Kitty, Badtz-Maru, My Melody, Dear Daniel, Keroppi and the others! You adventure with thousands of other players and computer-controlled characters in a virtual world full of cuteness and kawaii. HKO provides a brand new experience in gaming with built-in social media like video sharing and blogs. HKO is free to play!