As a first time buyer, can a customer sit on the bikes to test if they're comfortable?

yes dont be afraid of taller bikes theres things that can be done to lower them a bit they also settle a bit after you break them in

I took my Grandson there the other day, he was trying them out to see how comfortable they were, even a few he almost couldn't touch the ground right. Four wheel and even the two wheels. The only thing I can say is they're not like Bert's Motorcycle, they get very upset, I should know they told me that I couldn't do that. I told them that I'm taking my business elsewhere. They tried stopping me from leaving and tried apologizing.

Yep.

Yes, they encourage it

Yes you can :)