Thanks a lot for figuring that out, this helps me a lot! The minibike cranks might be much stronger than the children’s bicycle replacement part I found. I might use them on my 3m Pichler giraffe, where the crank needs replacing atm (Fun fact: I got it about half a year ago from somebody who works now in the old Pichler factory building in Karlsruhe. 
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The 90mm children’s bike cranks (Picture Below) were surprisingly durable for my purposes and are also made of steel, but that should not be considered a good reference because I am a very lightweight person. Their bigger problem is the poor manufacturing quality: I immediately returned one of them after purchasing because either the pedal threads had been drilled at a slight angle, or it was bend on arrival, which made riding feel a bit off.
I remember that my first ride on the giraffe with 90mm cranks felt scary because I expected it to be much harder to ride. However, after less than a minute, it turned out to be far more stable than a regular giraffe! I was even able to idle, hop, and ride backwards on it on my very first attempt. At the moment, I have replaced all the giraffe cranks used in the university sports course that I teach, and everybody learns to ride the giraffe directly with 90mm cranks. They genuinely seem to feel better.
As for the gear ratio, I think it should still stay within the effective wheel-size range that matches the short crank length. The largest wheel I would personally ride with 90mm cranks is a 24-inch wheel. Therefore, I would consider a 34T chainring to be my personal upper limit, because (34 / 28) × 20 inches ≈ 24 inches. I think, the 30:28 option you chose should be fine. (I never tried anything larger with 90mm cranks because even the 24-inch equivalent was already difficult for me to stop on, but as I said: I am a lightweight person, whereas a heavier person might have some advantages here).
One comment on the wheel you chose: to me it looks sturdy enough to withstand the forces from hopping on a giraffe, but I would recommend checking that it can be mounted the same way as the original wheel. All of my giraffes have a solid axle with axle nuts that are used to set the chain tension, as you can see in the picture below. The wheel you chose has a quick-release skewer, which should definitely be strong enough to support the load (I am always surprised that a quick-release skewer can be stronger than the hub axle itself. It has already saved me when the rear axle on my tandem bicycle broke! These things are incredibly strong). However, I would recommend checking whether the axle diameter is compatible with the existing axle nuts, in case your giraffe also uses them and you are switching from a solid axle. Quick-release hubs typically have a 10 mm axle diameter (although some special wheels use a 9.5 mm axle), whereas solid axles are usually 9 mm in diameter.
Regarding my other geared “speed giraffe” (Picture below), I can tell you that the 46:28 ratio is rideable once you have enough space and the courage to ride it at around 15 km/h. However, that machine is really just a gimmick and was never built for actual performance use. There is no possibility to idle, stop and ride backwards, and at that gear ratio hopping feels extraordinarily scary.
In case you are attending the RTB, I can bring the different modified giraffes so that you can give them a try. However, since Heidelberg and Chemnitz are quite far apart, I would be surprised to see you there.
