The other day, I realised I could only remember two of the kids' science fiction series that shaped my young mind.
First and foremost, beyond question, were the Danny Dunn books.
I loved them, not least because they made a solid attempt at getting the physics right.
When people get shrunk to the size of ants in practically any other sci-fi or fantasy story you care to name, they carry on with their lives more or less as normal in their scary new world of bus-sized cockroaches and bean-bag-sized blood cells, or whatever.
Which is wrong, for the same reason that it's wrong that Superman is so often able to take a firm grip of one end of a battleship or something and lift it bodily out of the water.
We can accept that normal physics doesn't apply to Superman himself, just as we can accept that absent-minded Professor Bullfinch in Danny Dunn and the Smallifying Machine has indeed managed to construct the eponymous Machine. But Superman doesn't magically make the battleship as tough as he is just by laying hands on it. The ship is still subject to normal physics, so when Clark grabs and lifts he should end up with two large handfuls of torn steel, and look like an idiot.
(Image Comics did this right at one point, with the new and clueless Mighty Man trying to lift a car by the bumper and, of course, just ripping the bumper off.)
Anyway, when Danny and company get shrunk, they find they can't walk any more. Because, of course, the acceleration due to gravity is still 9.8 metres per second squared, and if you're scaled down to a thousandth of what you were, that now looks like 9.8 kilometres per second squared.
So if you're standing up and tilt slightly forward with the intention of starting to walk, BANG you're on the ground. Just like an ant would be, if it tried to stand on its hind legs.
You suffer no damage, since scaling down makes you tougher in scale terms, but bipedal locomotion is completely out of the question unless your body and consciousness are accelerated by the same factor by which they've been shrunk.
Which, in the Dunn stories and in all of the crappy Incredible Voyage/Honey I Screwed Up The Physics Hollywood versions, they never have been.
(Warning! This sort of thing can lead to long conversations later in life about the stability of the Ringworld, which is even worse than prolonged Monty Python quoting when you're at a party and should be meeting girls.)
The other sci-fi(ish) series I could remember was Norman Hunter's immortal Professor Branestawm series, which takes a lot more liberties with physics but is plainly doing so in the service of humour. Branestawm is more of a wizard than a professor; he'd be perfectly at home in Unseen University.
(The Branestawm books, or at least the good editions of them, were also illustrated by nobody less than W. Heath Robinson!)
There was another series, though, that I just couldn't pin down. I could remember it featured a family adventuring around the galaxy in an old spaceship, with memory implanting machines to school the kids, and the spaceship needed its engines de-coked in at least one book... nope, no useful search strings arising from those memories.
(I include them here so that now someone who can only remember the de-coking, or indeed decoking, or decoked or decoke or coke engines spaceship books, will find this post.)
Anyway, considerable Google-bashing finally reminded me that those books were the Dragonfall 5 (or indeed but incorrectly Dragonfall Five, frustrated searchers!) series.
All three of these series are significantly dated these days, but I think that, in itself, has more educational value for the kind of nine-year-old who'll find them interesting. They're all out of print, too, but seem pretty easy to find on the used market, and should be available from any half-decent library.