Most such sites in Iceland are very easily accessible from the carparks, sometimes barely a few meters walk, sometimes less than 5 minutes.
Kerid had a rangers booth with a small entry fee. You then walk to a boarded viewing platform, from which you can sit and admire before walking the circumference of the crater, and or down to the waters edge.
In a world where so often, safety concerns overprotect and limit viewing or accessing natural wonders, Iceland is not so regulated. You will see photos here with no more than a slack rope suggesting you shouldn’t venture further. Personally, I’m not big on heights, so I avoid getting close to edges, but I would certainly keep a reign on the kids if you were doing this.
My visit was under half an hour, but you could rest longer and soak it in more as you wish.
