Foot Outside The Box

[starman]

I was hoping someone could give tell me if I right or wrong about this......

10 year old little lague game and I was coaching 3rd. Batter had 2 strikes.
The pitch was behind the batter, but the ump called strike-three because because the batter had a foot outside of the box.

I argued that there is no penalty for having a foot outisde the box unless the bat touches the ball. The ump tells me that anytime the batter's foot is outside the box, the pitch is a strike.