Have you/she thought of measuring penetration into one material at the same temperature but just at different distances, maybe 2m intervals? She could do 5 trials at each distance, average the penetration and go from there.
She would be able to make some great graphs and whatnot to explain her results, hypothesis could be that the arrow will show a linear relationship when comparing arrow penetration to the distance. Pretty easy to measure and would be fun to either prove or disprove her hypothesis. I would be pretty happy to see a project like this and I teach higher science than grade 6!
Andrew