The 21+ requirement on the dance clubs came fairly late in PI's history, following a major blow-up at Motion involving underage CPs getting into a big fight. In fairness, WDW had turned a blind eye to underage drinkers (especially CPs) in Motion for a while before that, really ever since Sunday/Thursday admission for CMs went to 7-night-a-week admission (the last year and a half to 2 years the clubs were open).* I think the change to HoB's booking process came even later, tho
While children were technically allowed in all the clubs except Mannequins for much of PI's existance, I don't recall it being an issue when the turnstiles were up (and, I suppose, before "Magic Your Way" became a buzzword)--everyone needed a ticket, and few parents were going to shel out $20 to get their kid in. I supposed the turnstiles served as a psychological barrier to bringing kids, as well. And even in the Island's latter days, children were so rare in any of the clubs other than the AdvClub that the issue seems pretty minor.
JT, I actually agree with you, any nighttime entertainment area should be at least 18+, preferably 21+. But I still don't see how a restaurant built around a literal shrine to tequila, a cigar bar, and Raglan are a "family-friendly" direction. 3rd party yes, family-friendly no.
* As an aside, and I know I am opening a can of worms here, but allowing CMs 7 nights a week of free access was probably the second biggest negative change to PI after the turnstiles being removed. A lot of the clubs transformed, not for the better. Motion became an underage drinking paradise for the residents of Vista Lay. Most of the shows at Comedy Warehouse became more "inside baseball" than the average tourist could understand. And the Adventurers Club was "Rocky Horrorized" to a point where it became a lot less funny and accessible to first-time visitors.