I used yarn in my stash - Bollicine Dolly. The yarn is terrible - it kept splitting, and I'd have to cut it and start further down the strand. I also didn't swatch, and just guessed at using size 4 needles. The ornament turned out the size of a softball - not exactly the size you want on a tree. Furthermore, I decided to knit the ornament in the round and stuff it with polyfil, rather than knit it flat and sew it around a styrofoam ball. (I actually started to knit it flat, but the yarn was really annoying with the splitting and I just got sick of it. Too bad too - I've never done colorwork flat, and a small project like this is a good way to try it out.) The result is rather lumpy, and not at all attractive.
See what I mean?
The kids have asked if they can use it as a ball. I think I'll let them.