Re: Too dangerous for children?
I am not aware of anything in aikido that would prevent a child from participating. Most youth-related precautions derived from sports are in the areas of strength training, drug abuse, and obscure/undiagnosed health conditions.
That said, I think the challenges of aikido for children lie in the application and philosophy of the art. Judo and karate make great initial martial arts leading into aikido, and may prove better arts for starting a child's martial education.