Either air fork or rigid fork… Springs like on the Riprock tend to be a waste of weight for kids weights so I’d go with the bigger tyres and rigid forks given a choice. You could change the forks if they don’t work out but
your talking £150
https://www.bike-discount.de/en/buy/f1rst-air-24-mtb-suspension-fork-51499?lang=en
Both have disk mounts so you can upgrade the brakes easily but both also have a freewheel not a cassette so you either take the drive-chain it comes with or need a new rear wheel/hub..which I guess will be a custom build with the fat rim???
Both have a long crank length 152mm for a 24″ bike… at least 10mm longer than it should be… (135-140)
Not sure about the height range but that Rip Rock weighs more than one of my Hardtails* (Maybe)
I’m pretty certain it weighs a LOT more than mine