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Are you specifically after discs? If not I'd be tempted to buy a whole, second-hand, rigid 24" MTB/hybrid from a cheap brand that doesn't hold its value but is basically sound (Decathlon/Rockrider, Carrera) and upgrade it with whatever you've got in your parts box. The components are often crap, but the frames are fine.
I did that with a (front sus) 24" Rockrider MTB and it turned into a nice chuckable bike. You're unlikely to get it really light, but you can go a fair way to that if you upgrade the finishing kit and go 1x? rather than the usual 3x7 that they come with. Proper short 1x cranks are usually the pinch point in terms of whether this approach is decent value vs. just buying something new.
My daughter's rapidly growing out of her 20" Frog Bike. I'd like to build her something like this Early Rider: https://earlyrider.com/products/seeker-24 from parts I already have lying around.
Are 24" kid-friendly framesets a thing? If so, what should I be searching for on Ebay?