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My kids are on the acolyte super short 8sp mtb group.
The R9 would be the microshift bits compatible with sora, but like I mentioned, I don't think they're much if any easier to work than the sora shifters. Maybe some bar top thumb shifters would do the trick for your son? Something like these might be easier to operate and will fit many drop bars. Not indexed, but friction is really easy to get used to. Otherwise, some thumbie pods with 23mm clamp dia for Shimano 9sp bar ends can be found and those bar ends will index really well for sora rear derailleur. Front is friction.
https://www.sjscycles.co.uk/gear-shifters/diacompe-ene-friction-thumbshifters-pair-silver/?geoc=US
He’s on drop bars. Does your kids have the R9’s? What would you suggest for easier shifting for my son?