It might be a good trail to start with a 3 yr old but probably not finish. If you're intent is to make it to the tank itself, probably not. There's no reason why you can't turn back if your 3 yr old gets tired or the trail becomes too much. The lower part is pretty flat and certainly well worth the walk.
No, this trail involves bouldering at many points
I wouldn't recommend for young children
I don't think I would recommend this one for someone that young, but maybe the calico hills trails would be a better option (fewer giant boulders to climb).
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