Reach Adjust on Shimano hydraulic brake levers for MTB flat bar
Reach adjust is though a small (2.5mm?) hex key on the 'inside' of the lever. The MT400's definitely have it.
The amount of reach adjustment is quite wide, On my MT400's, from memory you can adjust it down until the lever very nearly hits the bars before the brakes are fully applied, so will be as good as can be for a child. Higher end brakes have better performance and adjustments like free stroke, that might help those small hands, but the price/performance/reliability of the MT400's or 315's is impossible to beat.
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