Commish wrote:

For the folks who suggested rollers - I like that idea, however, where
I want it is down a slope. How much slope? My 4yos can sled down it
safely, but you have to stop them from the bushes at the bottom. I
wouldn't call it steep. But it's a little more than gentle....

Once on the rollers, how likely will it be to take off?

Very.

Stuff weighs more, sometimes MUCH more, going downhill than going up.

I refer you to an old movie: "The Gun." They got it UP the hill okay, but
going down it broke loose and cut a swath about fifteen feet wide through a
forest.