It's a personal choice either way, but my opinion is that these are a waste of money. The best way to build strength is to use your own bodyweight for strength training. You will end up functionally strong, not just with vanity muscles. Look to how the military trains. They don't use machines. Pushups, pull ups, mountain climbers, planks, tricep dips, lunges. That's all you need, and it's free. My husband is a career Marine and the only equipment he owns is a pullup bar and a set of 25lb free weights that he uses to keep his upper back/shoulders strong.
I would also not recommend having a 9 year old use weight machines. Kids should only engage in bodyweight exercises until they are around 13 or you can cause stunted growth and injury. Most kids don't have the discipline or ability to use proper form, and lifting heavy weights can lead to premature growth plate fusion. Ask your kid's doctor before you start a weight program, especially if they are on the small side for their ages.
A set of free weights and a bench is a much better investment.