After trying this with a few classes, I don't know why it never occurred to me to have them draw on colored paper, using the black and white charcoal. I did that for my second example, (above) and it turned out much better. But still, they are able to do the project with minimal whining. I've found it's important to give them only the soft charcoal pencils and not the hard ones, since the hard ones obviously don't make as dark of a mark.
But here are a couple of the first few to be finished. The later classes have drawn them on construction paper. I found the construction paper, being more fibrous, I guess, took the charcoal better and added more contrast to the shaded parts. But the white ones are very nice as well. I found I really had to get the kids to think SIMPLE! I coached them through drawing the center of the design, and then let them finish it however they liked. A lot of them wanted to go nuts with overlapping knotwork, which did not translate well when they tried to transfer it into the other sections.
This one was just simple enough, and she did great with the shading. So there ya go, Fourth Graders CAN do Celtic knotwork! :)