I wouldn't worry about it. I think squabbling is normal sibling behavior. Have you seen Shark Vs. Train? A hilarious book on the topic of who would win in different scenarios. It's hilarious!
I once had a critique at a conference of a PB about a kid with a temper tantrum. The author who critiqued basically felt the same, it wasn't the most appropriate topic for a PB, or at least the way I resolved it. BUT, my CPS love it, and so did a former editor who definitely encouraged me to send it out. This industry is SO subjective. If you like it, keep going with it and see where it goes.
Good luck!