Unfortunately I cannot think of any good King Arthur movies suitable for the under 12s. "Excalibur" was fun but wasn't suitable. Neither was "Monty Python and the Holy Grail". My son, aged 8 at the time, like "A Knight's Tale" and we had the jousting sets out for a month or so after he saw it.
There's a REALLY old movie with Bing Crosby in it called Connecticut Yankee in King Arthur's Court that I can remember watching when I was very little. It wasn't violent; it was funny and had singing in it, but I don't remember too well because I haven't seen it since I was a kid. I say that, but now I'm humming one of the songs, so I guess I do remember!
As for Romans, I watched Ben Hur all the time because my older brother loved it, but I don't really
recommend it for kids neccesarily. It's very long, and sometimes a bit intense for a very young child (I was slightly traumatized by certain scenes). But it will certainly generate interest in roman chariots and galleys.