Field Balk

[Kidcoach]

Base umpire calls "Field Balk" because firstbaseman is straddle the bag and has one foot in foul territory. No one I talk to has heard of this, nor has the Plate Umpire. Base Ump says all players must have both feet in field of play at pitch. I point out that the catcher obviously doesn't, and in my opinion, one foot puts the player on the field. I have been looking, in vain, for the answer. Appreciate any help.....