This is one of the few films that my super sensitive 4 year old will actually watch in its entirety. That doesn't mean there aren't some tense moments. But for whatever reason, the tension isn't enough to make my son want to turn it off. My theory is that he is so smitten with Kiki that he doesn't want to take his eyes off of her, so keeps watching. The cat character in the film is also very charming.
As a parent, what I LOVE about it is that there are no guns or aggression, there is no villain or "bad guy" and the main characters all have good relationships with the adults they meet.
As parent of a son, I also love that this film depicts a girl who is in control of her own life, and overcomes obstacles to eventually become the one who does the rescuing in the film's climax.
I also love the slow pace of the film, which lets my son digest everything that he is watching.
The final scene, in which a boy is holding onto a rope while suspended 100 feet in the air is quite tense, however, I think the younger kids might not realize how dangerous the situation is. The kids who are old enough to understand that the boy's life is in danger are probably able to handle the depiction of peril. The moment of rescue, is a beautiful moment of animation.
This is possibly my favorite Miyazaki film (along with My Neighbor Totoro), so not only will your kid enjoy it, but you as a parent will want to sit down and watch too.