Having just done them, it's not for a 6 yo. It's super interesting but even for adults you have to be interested in history. It has an audio guide that I used a little but skipped most (not a fan of them, I'd rather read signs of what I'm interested in.)
The imperial war museum was interesting too but I wouldn't call it kid friendly either, though there were some kids there.
@Haley R if you end up at Piccadilly circus go in the
Lego store, they have some cool builds. I do hope they use ac in the summer though, a lot of places were awful and it wasn't even that warm out.
There's scaffolding up at Kensington at the front so there really wasn't anything to see. No idea how long it's there, hadn't looked to see what was happening.