Party Bus Ages

My wonderful fiance and I are currently having a small disagreement on party bus invites.

I have invited my bridesmaids and their dates/SOs. One BM is going to have a 4 month old. And one BM is the mother of our ring bearers. I don't feel comfortable telling the 4 month old's parents that they can't bring him. Up to their discretion. My theory is if they are itty bittys, that's their parents call.

The party bus is more of a requirement as we are going out to take photos. I mentioned to my fiance that we may need to make exceptions for itty bittys and our ring bearers and he is not here for it.
To be fair, our ring bearers live in a very liberal household. They won't be exposed to anything they haven't been before. [Language or witnessing drinking.] I think it's a little rude to tell MOH's husband he isn't invited on the bus like everyone else's husbands and wives because we asked his children to come to the wedding.
I guess I just don't see the harm in allowing little babies and 2 kids that are in the wedding party on the bus if it will make it easier on our VIPs.
Ideally, ring bearers grandparents are attending and can go with them. But as a non parent, asking the wedding party to separate themselves from their 3 or 4 month old babies seems wrong?
Am I wrong? Particularly biased toward allowing my BFF to do whatever she needs to do?
We're also arguing because he only wants to allow 1 of our 3 ushers on the bus with his wife. For some reason his 24 year old sister and her boyfriend shouldn't be on the bus. Nor his 20 year old brother and his 19 year old girlfriend.
Aren't ushers usually in photos? It would be odd to only have one usher in photos.
Plus he wanted to invite this usher and his wife onto the bus before he decided he wanted him to be an usher.
Help me.