I'm not sure about a helmet/head covering but for the knight, I'd make a cape, a pair of elastic waist pants (pajama patterns are nice), and this
, out of fun metallic fabric to simulate a plate of armor. I know the pattern is for a dress but it's such a versatile piece that children can get on and off with little help and the pattern is for a great range of sizes. My daughter was a pink cat for Halloween last year and I found some fantastic pink fur and that was the top of her costume, along with pants with a tail sewn in, and a headband covered in the fabric with ears on top. Done. Heck, I'd even consider lengthening the pattern and making a princess something or other out of it and pairing it with wide legged pants out of the same flowing material.
(I also like it because I can stick the appropriate length sleeves under it in a matching color.)