Considering hugelkultur at my preschool but there would be no way to keep them from climbing all over them! Anybody have experience with kids and this kind of raised bed?

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.

My twins are preschool age now, in fact. I give them digging places and they know not to dig in the gardens, I make the paths visually very clear and they're pretty good about staying on them. They helped build the hugels, so they know not to climb on them.