How to say "Peekaboo" when playing hide and seek
It seems there's not a suitable Chinese name for the game 'Peekaboo'.
Someone says its name should be 躲躲猫, but I'm not sure.
Anyway, if you just want to know what should be said when you show up in a sudden in the game, you can say any modal particles such as "啊!", "哈!" etc.

BTW: In Chinese 捉迷藏 usually means a game played by several children. One child is selected, cover eyes and make a countdown, and the others hide themselves in the period of the countdown. And then the child selected try to find out all other children.
捉迷藏 also can be said as 躲猫猫, 藏猫猫.