I did this climb with my 4-year old son. It was extremely crowded (8am) and getting insanely and dangerously crowded when we left around 10.30am. You park/walk to the Diamond Head State Monument, pay your fees and start the paved walkway. Take double the amount of water you think you need as there is little shade and it gets very, very hot, even early in the day. This is a very popular walk, so there were masses of people of all ages going up and down, some people race each other to the top. The paved walkway stops and then the path gets rocky and uneven and narrow up the mountain. Then you get to 2 extremely steep staircases (with only one guardrail on the side), through very narrow badly smelling badly lit tunnels, then up another steep spiral dirty rusty staircase, very very narrow and steep and then through another tunnel and you get to the top where you have to crawl through an opening to get to the lookout. Again extremely crowded with people hustling for position. No rangers along the way. No one to provide first aid, nowhere but 2 places to step aside and rest or sit down. Nowhere to get out of the way for faster people. It is a long crowded procession up and down the hill. One way up one way down. Not a pleasant experience.
If you are claustrophobic and/or scared of heights, you will not make the last bit. It was intensely uncomfortable to squeeze through with so many people.
My son freaked on the staircases on the way down - badly so and there was nowhere for us to go, take a breather and regroup and also no guardrail to hold onto, as people were coming up the other way where the only rail was. People were impatient and pushing through and we needed to go slow and steady and carefully, as my son was so frightened, he barely made it back down. It was that steep and scary. He was shaking at the bottom.
I would not go up again with a young child, as it is way too steep and scary at the end. If I would have known how bad it was, I would have skipped this until he was older.
I also didn't think it was particularly safe, as there was no ranger on site to help or even direct the people and manage crowds, as the lookout only really holds around 20-30 and I counted 350 people on the way up when we were walking down and one lady in need of first need passed out on the side of the path.
Grateful for the toilets and the great yellow food van at the bottom which serves lovely food.
Go with hiking shoes and lots of water and leave young children at home and go as early as you possibly can!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.