You guys amuse me, .. like you will have any say in what team your child eventually roots for, any more than their favorite car, girl, music, movie, etc.
When they are at an impressionable age,(~10-15) and one team is dominant, or very successful, they may choose to cheer for that team.
They may not, .. they may always choose to root for the Cowboys, .. but if they don't, it is their decision.
I was fortunate that both of my boys were impressionable during the early to mid 90's.
I'm like Hos though, .. my father, and my whole family are steeler fans.
Being the black sheep of my family, I could not dictate to my kids who their favorite team will be.