These were done with my pre-K class of 4 year olds.
I had them trace a circle and after we looked at photos of snowflakes and talked about them a bit they had to draw their own. They used light blue and gray crayons at first - this avoids the "I can't see what I'm drawing" comments that are inevitable if you give them white. Trust me.
When they were happy with their design we went over their lines with white crayon - rubbing down super hard. (I find they draw so lightly and timidly sometimes. I am constantly trying to get them to make firm clear lines. Why is that? I blame markers.)
Next I let them loose with the water colors. I had planned on them just using blue and purple but they had other ideas and that was fine with me.