I don't know, I find ROTS too dark for my own tastes.
It was very dark, but then IMO it wasn't any darker than the old Warner Brother's or Tom And Jerry cartoons many of saw as children.
I definitely wouldn't go that far. The cartoons don't have the same feel at all IMO, and the fact they're cartoons not live action also diminishes their impact. Even as a kid, cartoons were cartoons and I knew never to take them seriously. A live-action film was always more effective in making me suspend disbelief and therefore become affected by it. For example, when I saw Superman III - obviously something I knew was fictional and cartoonish in nature - I got frightened when the computer merged with the evil sister at the end.
Now, I'm not saying fear
per se is necessarily a bad thing for a kid to experience from a film, but it can be taken too far. Given that the tone of ROTS is genuinely dark in the second half, especially the killing of the younglings bit, I think that takes the film away from particularly young children. Once they're a little older, fine. The scene with Anakin in the lava is also a bit much for young kiddies, I think.