With LHBs still a rarety, the positioning of the 1B is way down my priority list when it comes to in-game management.
However, at 10U, I'm OK with her playing even or a step or two behind the bag with no one on. The slower the pitcher, the farther I'd have her off the line. If a 10U-C level player can drop a bunt down 1B line - my hat's off to them - she can have 1B. Bunting just isn't part of the game yet for us - it's more about being able to cover 1B than cover a bunt.
Left hand batters a rarity? Our team (12U) has 10 lefties. Gotta know the girls love it when they see 1B playing all the way back by the bag.