Grindsbrook is generally straightforward walking, albeit on a stoney path, until close to the top where it becomes rocky and hands-on in places.
Easy walking, easy navigation at the approach to Grindsbrook
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Path alongside lower Grindsbrook, only very slightly stoney
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Getting rockier about half-way up
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The final push, not too steep but the path has disappeared
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Grindsbrook is a fine way to reach Kinder, a challenge, maybe an adventure, but not life-threatening. My guess would be that your son will muchly enjoy it. Hope so!