What if you made a visually clear path of stepping stones over it? Also a short
fence to clearly show where not to step. It could be a fun feature, climb up the hill and down the other side. You could even put a slide right on it!
I always find it easier and more effective to tell preschoolers what to do rather than what not to do. Any sentence starting with "don't" is pretty much useless with three and four year olds.