BlueSky
I would have left if that was the rule and we preferred sitting upstairs.
... that's your choice.
The manager clearly stated his.
Sometimes adults want to sit quietly over a coffee - and frankly, it can be quite nerve-wracking when youngsters, because they are naturally exuberant, are running around, bumping into tables.
Perhaps if parents had better control of their offspring things might be different, but so many don't.
The management made the rule for a reason - probably having experienced the disruptive behaviour of some kids.
... and some adults might have energetic and noisy kids at home, why shouldn't they be allowed a space to relax for those - possibly rare - moments when they can unwind?