Junior's bike has SLX 675 brakes (and original pads) which are very on/off, no modulation.
Interestingly I have exactly the same series SLX (and pads) which are nice and gradual. Both sets were properly bedded-in from new.
How can I improve Junior's brakes? A fresh set of Shimano pads or perhaps a different brand? And/or don't bed them in?
Are the original pads sintered? I've got two sets of XT's, both came fitted with sintered pads from new. Organic pads will change the initial bite.
shimano brakes seem to be really variable, even from brand new. I've only owned one set of my own but used a few on other bikes (hire/demo etc). They seem to run all the way from "massively grabby" to "good power but not exactly subtle". I assume it's a bleeding thing, though if it is, then presumably the well-bled ones are the grabbiest.
Try swapping the pads from your brakes to his. If that doesn't make any difference, try swapping the rotors. After that, I would try sanding the rotors and trying a different compound pad.
Check the rotor is tight.
Squeeze the lever less hard?
Said only partly in jest. Remember the first time you had discs or v brakes?
Adjust the throw of the lever so that the bite point is closer into the bar. I presume your son has smaller hands than you do...
ok good chat. Rotors are tight. I swapped pads from mine to his and reduced his lever throw.
The feel is now nice and gradual on both bikes, presumably because pads and rotors are not mated. I'm not going to bed them in, in the hope that they will now maintain their current feel.
Hydraulic pistons will/should self adjust to a specific spacing. That spacing is determined by the piston seals and is not adjustable, so if one brake has had pad(s) too close to the rotor then have, or have had, an issue with a stuck/stiff piston. I'd not be surprised if the problem returns after a little use. Could also be a set-up issue - you may find that the caliper is misaligned on the youngster's bike.