This does sound extreme, but we have a dad who is a supporter and he said "the first football match the kids go to is X team" after that they can do what they like.
Which was fair enough.
It took a while because they are a top premiere league team so took a couple of seasons to get the required 4 tickets together.
So I get it. But we had no rival grandad getting involved.
You know they are your husband's kids your dad had his turn with his kids. I would say this to your dad.
But they both need to grow up and not wind each other up.
Your kids might end up just being at best fair weather fans. After all our passion and planning one of ours couldn't care less about league football, even though he does have "the team x" as a favourite I guess if asked, the other loves football but isn't a knob about it.
Explain to husband making your kids rabid supporters might also not be the best idea. It can get aggressively tribal and stunt sportsmanship development, actually just generally stunt all sorts of development.
So yeah, see your husband's point but everyone needs to tone it done a bit don't you think?