I don't think most kids would pick up on that kind of nuance (or even most adults), but I agree there's a valid interpretation that you're pledging allegience to the Constitution – the Republic – and thus "indivisibility, liberty, and justice". That is: you remain allegiant to the Constitution. But the current pledge has so much wrong with it that it's cult-like.

• Obviously get "under God" the hell out of there. Cold War-era reactionary trash.
• There's no reason to assume from the literal text that what I said is true. Why not just focus on the principles?
• It's a waste of time for kids to recite a dumb pledge they barely understand; granted they can't force you and a lot of schools IIRC don't do that anymore.
• Even if the interpretation is true, why should this specific system of government be so glorified?
• Get it the fuck out of there. It was introduced 100 years after the formation of the US by a Civil War officer as propaganda for children – probably paranoid out of his fucking mind after the South seceded. There's no reason kids can't learn to think for themselves when they're ready to actually understand these ideas.