what is your strike zone?

[C rabby]

I am umpiring 9-10 year olds, and am curious how liberal I should be with regard to the strike zone. I did a game last night; and was not giving the outside off the corner pitch to the pitcher. I am wondering if I should have; many of those pitches were close to, but not on the corner.
Also, what is a good way to not call the high pitch as a strike? I am seeing a lot of small (really small!) players and it is hard to judge sometimes. Thanks in advance!