When it comes to kids, you have to choose Kirby. Kirby Nightmare in Dream Land, and Kirby and the amazing Mirror are great for kids. and if he or she is fairly new at games, Kirby is also the way to go. The Kirby series was originally created to be a learning experience for those new to games. Kirby's Dream Land for GB was an easy game which was good for beginners. Kirby and the amazing mirror is a bit harder than most Kirby games, but it's still pretty straight forward.
Though I tend to disagree with the Mario and Pokemon games. Pokemon ruby/sapphire were the worst, and hardest of the RPGs, and Pokemon pinball ruby/sapphire was really hard. Some Mario games are really tricky too.
are you kidding? those are insanely difficult for hardcore gamers and adults? i _love_ them, but i never actually recommend them to anyone, let alone little kids. you can play them on easy, but then its really not the same game.
edit: there are scenes in megaman zero where, soldiers get cut down with a laser. the US versions were censored with less blood, but its still pretty shocking, even to an adult.