I had a similar issue with my eldest’s Hotrock 20, it came with 6speed tourney, a twist shifter and a screw-on freehub, so I went and rebuilt the rear wheel with a Claris cassette hub (130mm spaced) and fitted 9 speed with an old LX trigger…
She never really liked or got on with the trigger shifter, so eventually I located a 9 speed SRAM Rocket halfpipe, this she is well able operate…
In retrospect I’d probably have been as well leaving it with the original drivetrain, she got a smidge more range and slightly less weight for quite a bit of faffing.
TBH, bike snobbery aside, I actually think Gripshifters suit more kids with their diddy hands and developing bike skills, they’re quite simple to operate and pretty robust…
In your position OP I’d maybe have gone for a seven speed cassette (say a 12-30 perhaps?) and looked for an older design SRAM 2:1 gripshift, any 7/8/9 speed shimano mech you like should work with that.