.. And then the family complain to the hotel - "We will never come back to a hotel that treats us rudely and hates our kids".
It isn't a practical solution. A simple age ban is a simpler rule to enforce that everyone can understand and causes less problems and confrontations.
The scenario you outlined is a rather small subset of possible outcomes however uofron ban of kids from lounge will with hight probability ensure that families will not come to the property. Not only that, i personally would try to avoid such property and chain when traveling alone too and i bet many parents would do too. Most parents do try hard to make sure kids behave.