I’m 6’6” tall, and recently chartered a ‘20 FP Lucia, Shelly Kay out of BVIYC. I can’t speak for the berths in the lagoon exactly, but I can for the Lucia. I’ve day sailed a lagoon 40 but never really went below. I like Cats as I seem to end up with better headroom than Mono’s.
On The FP I had no troubles in any space with headroom, or length in the “owners suite” aft berth. Same on the cockpit/salon spaces in the lagoon.The forward saloon windows are vertical in the lagoons, carrying the ample headroom farther forward than the “faster” raked forward windows of the Fontaine pajot. I like the FP’s lines better, but for livability the lagoon undoubtedly has a bit more volume. But how often do you stand that far forward? A lot of that space is over couches/nav stations…
I’d imagine either would be just fine for your boys, and they can be in charge of prepping and stowing the main, as sometimes the zips on the top of the sail bag are hard to reach for normal sized people.