Cant blame the kid. Don't think I'd call it brainwashing exactly. Most kids will repeat what they hear. I mean how many kids like the sports team of their parent, and dislike a rival team? Do they have reasons? No. They've only been on the planet for like 10 years and maybe watched one game they understand. But if mom and/or dad like Team A and hate Team C that kid will likely say the same. What's important is that the kid learns to form his own opinions and thoughts and not just rely on what the adults around him say with not further follow up.