We walk a lot but we considered it walkable. I'd give yourself a half hour or more to walk up (from the Cathedral) but you may get there much faster. You don't need a guide; it's slightly confusing following the directions in Rick's book but we made it without too much trouble. If the walk seems too long, or you're worried about getting lost, you can easily hop in a taxi and be there in not much over five minutes.
I didn't think it was unsafe in any way. The area is mostly expensive restaurants (that sell the view).
We got there early so wandered a little and came back closer to sunset. It's a nice scene from the time the sun gets low and reddish until the sky gets dark and you see the Alhambra lit up.
There's a bar close to the cathedral called Antequerra (also another Antequerra II not far away). When you order a drink they give you a toasted ham and cheese sandwich with fries and some fritos - really a whole meal that your 7 yr old may really enjoy. I think your 7 year old can go in before 9pm; we didn't have a kid in tow so don't know Spain's rules for sure.