I've been twice for ds's 10 and 11 birthdays. Advice:
With a 12yo, I'd plan to stay late, because after about 6-7pm almost all the bored toddlers vanish and the lack of buggies makes it much more spacious. We took 4 hours both times, but might have stayed longer without a 6yo in tow (first time) or being kicked out at 10 (second time).
It's in the middle of nowhere, 15 min free themed bus ride from Watford Jn. It's bumpy with twists and turns so not great for the car-sick.
Quite a lot of standing around, airport-type security into a main hall where there are 2 cafes, then wander to the main shop and can enter the start - go past Harry's room under the stairs to a hall where someone goes on about how the movies are loved round the world, then move into a lecture theatre for a bit more being talked at, then finally they show the doors to the Great Hall and kids with a birthday get to open it. At which point you're in the great hall and while the guide says a bit of stuff from there you are able to wander.
From around 4pm you can get let in early as there aren't crowds to get in.
The cafe before entry does very nice icecream with glitter and an inch-long chocolate frog. They damn well should be nice at a fiver for an admittedly large cone, but the rest of the cakes etc were typical nice-coffee-shop.
It's worth seeing at least the first 3 films shortly before you go, so you recognjse the items. Loads to see in the first hall of Higwarts sets, then can queue to dress up in robes and have photos taken like posters and can fly on a broomstick and see yourself on video. Both can be purchased for some huge amount of money like £30. It may be worth accidentally moving on and claiming you can't go backwards through the place (you can. We did, a lot) to avoid kids wanting you to buy them.
Then Forbidden Forest. Bear right if anyone wants to avoid Aragog, but it's very well done.
Then wand shop, Platform 9 3/4, opportunities to do the trolley-in-wall photo, another shop, and then the main cafe.
One butter beer between you as its foul - sickly sweet butterscotch but fizzy. The food is OK but expensive, but vital note for those with young kids - you cannot buy portions of chips, only whole meals.
So I'd recommend maybe kids have icecream before or after entering, then one token butterbeer with a picnic. Then there's outside, and then the Bank, followed by the more 'how we actually did effects' part, which may not have the same 'wow' factor as the main sets but is fascinating for anyone interested in filmmaking.
So don't panic if much of your time is taken by the part before the cafe.
The shop has huge amounts of merchandise but when we went it only had crap unnerving cuddly Nifflers. Lego is similar price to elsewhere so a good thing to buy. The various notepads and pens and clothes are horribly expensive and no better than Poundland. The official chocolate frogs are £9.99 so definitely don't tell your child they can have one for every year of their age... The wooden wands are £30 and nice bits of wood, but basically just a stick...
So yes it's expensive, but with some planning you can avoid extra costs, and we did feel that yes, the credit card might wince but we felt satisfied rather than at all ripped off.
Except the chips. (autistic kids who will eat nothing else when away from home...)
Also their accessibility and staff training is excellent, in particular the "help we've lost our child who won't answer even if he hears his name' protocol. Staff are all fans and will tell you anecdotes about the films.
I'd certainly take it over a theme park which costs as much and you spend most of the day queuing.