That’s actually a pretty nice problem to have.
Yes, you can use the lounges on arrival as long as you have your onward boarding pass. Cathay is usually fine with lounge access during a long transit. Plenty of people shower, eat, relax a bit, then head into the city and come back later. You shouldn’t have issues entering again before your next flight, as long as it’s the same day connection.
With 8 to 9 hours and a kid, I’d keep it simple. Airport Express into Hong Kong station is fast and easy. From there you can walk or short taxi to the Central ferry pier. Star Ferry over to Tsim Sha Tsui is always fun for kids and the promenade views are great. Grab dim sum or roast goose on the island side after you come back.
Just keep an eye on timing and aim to be back at the airport at least 2 hours before departure so you can enjoy The Pier or The Wing properly. The cabanas at The Pier are amazing if you can snag one.
Honestly HKG is one of the best airports in the world for a long layover like this.
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