In this context it's important to know that there are two museums at Bletchley and they aren't entirely co-operative with each other. There's The National Museum of Computing https://www.tnmoc.org/ and there's also Bletchley Park's museum https://bletchleypark.org.uk/ these are both on the same physical site, in the grounds of the Bletchley Park stately home because that's where Ultra happened in WW2
Both museums do have some kid-friendly activities, but their focuses are very different. You might wish to visit one, or both, and you should figure that out before going as they aren't even necessarily open at the same time.
TNMOC is about Computers generally, but has some exhibits about Enigma including a Bombe reconstruction and then of course Colossus - to break the Lorenz cipher, only exists due to the war and would have been at Bletchley. If you don't much care about Computers that's not too interesting, maybe worth a half hour if you've time.
The main Bletchley Park museum is about Codebreaking and particularly Ultra, the secret project to break German codes, most famously Enigma but also Lorenz and others, at Bletchley Park in WW2. It has some exhibits about spycraft, and a lot more about the practical undertaking of this codebreaking. Who are these people, what are they all doing here, what was their life like? If you care about the people you will want to visit this museum, but it has relatively little about the technical nuances of what was done.
It's probably gone now, but rather bizarrely there was a harrier jump jet parked on a patch of green grass when I was last there some years ago. Sort of "dumped, pending working out how to deal with it" which made me feel like nobody runs the place and everyone runs the place. "Joe said I could" -"oh OK put it over there then"
Another thing about the place is that modern archeology doesn't favour restoration of everything. Limited amounts of structural change happen. So a lot of the site looks like dismal ww2 badly poured concrete, not well looked after. Since GCHQ occupied the space for some time, some of it is probably post war, and historically relevant to different stories.
There was a whole bunch of random historical stuff, like a cinema museum with a collection of old projectors and a model railway at one point. It was really fun. Those had all gone last time I was there.
Also when I was last there, The National Museum Of Computing had free parking, but Bletchley Park museum charged for their car park, even though it is right next door. You could park at the computing museum car park and visit either or both.
Both museums do have some kid-friendly activities, but their focuses are very different. You might wish to visit one, or both, and you should figure that out before going as they aren't even necessarily open at the same time.
TNMOC is about Computers generally, but has some exhibits about Enigma including a Bombe reconstruction and then of course Colossus - to break the Lorenz cipher, only exists due to the war and would have been at Bletchley. If you don't much care about Computers that's not too interesting, maybe worth a half hour if you've time.
The main Bletchley Park museum is about Codebreaking and particularly Ultra, the secret project to break German codes, most famously Enigma but also Lorenz and others, at Bletchley Park in WW2. It has some exhibits about spycraft, and a lot more about the practical undertaking of this codebreaking. Who are these people, what are they all doing here, what was their life like? If you care about the people you will want to visit this museum, but it has relatively little about the technical nuances of what was done.