I am not saying that the hotel should be open so people on their own without kids can go in, I'm saying the hotel should be open so families from other hotels can go in.
Why is a non-existent 40 year old man being brought up...
Okay, that's fair enough. MakoMania mentioned a 40 year old man in the Hotel and you responded to that post, so I thought we were talking about enthusiasts (some of whom might be 40) having a quick explore of the Hotel, sans children. That is what I thought might be a bit odd to parents with very young children.
As for the rule regarding families with young children from the other Hotels not being allowed in, I'd imagine this is their way of trying to encourage these families to stay at the CBEEBIES hotel to begin with. If the parents initially don't want to pay the extortionate prices, but their children really want to watch the entertainment the new Hotel has on offer - entertainment that they won't be eligible to watch if they stay at the other accomodation - then the parents may feel somewhat forced to reconsider. More money for Merlin.