Astronaut, and by extension a fighter pilot. (growing up, all but one of the astronauts had been test pilots)
As a very small boy, like 4 or under, I wanted to be a firefighter, and also a garbageman, because they got to ride around and hang on the back of the trucks, which I thought was unspeakably cool.
Young child: an auto mechanic (or “car fix-it man” as I expressed it).
2nd grade thru high school: an astonomer (which I visualized as involving SEEING astronomical beauty and studying about it etc; I had no idea math was heavily involved, nor that astronomers seldom spend their [del]days[/del] nights peering into telescopes.
A science fiction writer.
Bolded above, was me – self and a large number of other British boys of my (“Baby-Boomer”) generation. “Driver” in Brit-speak, as opposed to “engineer” – of a steam loco, of course: until the late 1950s, Britain’s rail system was overwhelmingly steam-worked.
(In later years, I tried the preliminary stages of that same thing, on a preserved steam railway: quickly found that I had no aptitude for it at all.)
It is a great job. Here’s the crane I operated in my younger days. It was mounted on this vessel (ship in the foreground gives a good size reference). I did this for a few years and it was in some ways an awesome job (fantastic pay, great view of the ocean).
As to the OP’s question; my dream job when I was a kid involved a Ferrari, solving crimes and a black friend with a helicopter.
USN F-4 (later, F-14) pilot.
A marine biologist, because I loved whales and dolphins.
In middle and high school, I wanted to be a professional basketball player. I dramatically overestimated my ability to actually play basketball, though.
Fighter pilot, a dream that has never quite gone entirely away. 
Chemist. In those bygone days, chemistry sets actually contained chemicals that could make interesting stuff, like SO2 and H2S (and you mix them and see what happens) as well as chemical gardens (thiocyanate, if I recall). There was an alcohol burner too.
Zoo veterinarian. From maybe 1rst or 2nd grade until perhaps 11-12.
Capturing crocodiles in the Amazon.
Archaeologist (this was decades before Indiana Jones).
I always thought Siskel & Ebert had the best jobs in the world. This was before I knew they both also ‘wrote’ reviews for newspapers and thought they just had the TV show.
I thought it would be awesome to just watch new movies every week then go on a show and give your opinion about it.
This is the same as me. Add in Secret Service Agent, but the reasoning was exactly the same.
Paleontologist, since I was about 3 years old.
Pilot.
A cartoonist.
Unfortunately where I lived, it was a one cartoonist town.
[ul]
[li]Astronaut[/li][li]Firefighter[/li][li]Playboy photographer[/li][/ul]
My nephew told me years ago that he wanted to be a garbage man when he grew up. When I asked him why, he told me it was because they only work one day a week. My nephew; always thinking.
Astronaut.
Nobel Prize winner for Chemistry. I wanted to be like Madame Curie, really. Haven’t gotten the Nobel yet, but I did get a couple of degrees in Chemistry, one of them abroad.
