balk or no

[JohmPMiller]

I was doing the bases in a USSSA game (10 yr olds) with a man on first. As the pitcher delivered the pitch the PU called balk for the first baseman having at least one foot in foul territory. I know all players except the catcher is supposed to be in fair territory but I didn't know this was a balk. He did this later when the third baseman did the same thing with R3 on. Is this correct. I was watching the pitcher for balk moves and did not see what happened on first or third as the pitcher pitched.