I used to run a drinks-only no food pub and said no under 18s after 7pm. The backlash was loud and angry, particularly online, but takings actually went up steadily once we did it.
Children don't spend money, parents rarely purchase more than one soft drink for the kids (if that, hordes used to bring their own squash in kids bottles, their own snacks etc), they take up space that could be used by adults, and put off groups of drinkers and lone adults wanting some peace. Plus the tension we used to feel when the parents were clearly starting to get drunk was uncomfortable. Trying to cut people off due to having children in their care is an unenviable task. As is telling parents after a few drinks they need to keep a better eye on their children!
There are many pubs where children are welcome - think the hungry horse model with food, kids menus and play areas.
Pubs which do not serve food are highly unlikely to be suitable for children, particularly younger children under 10. Pubs tolerate them as they are struggling, but I genuinely feel most drinkers pubs would do better without allowing them.
It's particularly annoying for those parents who have bothered to get babysitters to have an adult evening and then end up listening to crying babies, screeching toddlers and iPad games anyway.
It would be a shame if cocktail bars and nightclubs become the only places for adults to go without children. Lots of adults want a low-key space they can go to without kids, where there isn't music blasting etc. Quiet pubs can be a sanctuary.