https://en.wikipedia.org/wiki/Fischertechnik
We had those in primary school and I think it was electronically more advanced then Lego was at that time (they have added a CPU unit in 1980). We actually built a caterpillar based vehicle avoiding the obstacles using a photosensor in 8th grade, so this would be 1989/90. At that time Lego was just a toy (or maybe the Lego Technic wasnt available where I was living, I am just looking at wiki and it looks it was introduced in 1977 and they had a motor but no electronic counterparts yet) and I think Fishertechnik was mostly used in schools due to their electronic parts (at least in my country, I dont know for Germany).
I got the Fishertechnik traffic light working on the family Apple IIGS at something like age 8 or 9. Code and all.
This was back before things like easily accessible Web or smartphones and I had no consoles. So bashing my head against a huge problem for dozens of hours was surprisingly easy.
I miss that kind of context.
https://en.wikipedia.org/wiki/K%27Nex
Capsela and K'Nex were both great. I absolutely loved capsela as a kid. Capsela was an incredibly satisfying toy to play with and it was simple to put together. Relatively small changes created significantly different machines. Understanding the toy made it more fun to play with, but it was fun with almost no understanding too. Playing with it increased understanding.
This is not from experience, but what I expect the answer is for older kids:
A 3d printer with help understanding how to mechanize is probably best.
Adafruit is probably one of the better websites for acquiring interesting parts:
Maybe what you're looking for: https://www.adafruit.com/category/227
It's hard to give a better answer without knowing age/budget/goal/etc.
Fun fact: first line for creating contact lenses was build using this system.
They have en e-shop (although quite poorly translated) and it looks like they are shipping within EU to US and to some other countries: https://eshop.merkurtoys.cz/
(I am not affiliated just used to like these things as a kid)
Nowadays I'd just get a cheap FDM printer and fasteners, though
I resisted getting a 3d printer for a long long time. This is coming from a man with a fully kitted-out woodworking shop, 2 CNC machines, welding equipment and a lot more.
I didn't see the 3d printer as being for anything besides miniature figurines and cheap tat, because that's 99 percent of what you see people printing. But it's SO useful. Adapters, enclosures, brackets, 3d printers take care of solving all those little niggly bits that are really hard to fabricate by traditional methods.
I recently upgraded to a Bambu X1 Carbon and highly recommend it. I also have a cheap flashforge resin printer which is great.
Two things I've used my printers to make, that have ended up as actual products; a guitar pedal enclosure FDM printed box with an open top, I used aluminium PCB panels from jlcpcb.com as the front panels. The result is a very professional and sleek looking design that's tough as hell. The other one is transparent push buttons for a Eurorack synth module I'm working on. I used my resin printer for this, and it allows me to make a custom button layout that sits on top of mini tact switches with backlights, and gives a user interface similar to custom-made carbon membrane buttons. - similar to https://en.wikipedia.org/wiki/Silicone_rubber_keypad
You can do a lot of stuff with CAD/CAM before even getting to the 3d printing stage.
For adapting kids toys to this purpose, there's also K'nex.
If you use the right screws you can even add some adjustability.
https://mbsitem.co.uk/product-category/mb-building-kit-syste...