Elsewhere I’ve posted about a problem that developed in my little yard as a consequence of applying too much moisture to the quarter-inch or so of Arizona Rock & Mineral Yard Mix: in my quest to get everything thoroughly saturated, I penetrated the sheet of 1/4" plywood I have as a base in the area. It could have been worse had I not used a product with smooth top and bottom, but it still sagged enough that a 30-inch section of track bowed up in the middle, ballast and all. I got good, if conflicting, advice from our friends here about whether to fix it or start over (I’ve started over, largely because this is a little interchange track and needs to be dead level and smooth for magnetic uncoupling). Apply my experience as you like, keeping in mind that it took days for the problem to show up and more days to partly recover on its own.
I concur with the comments about designing the yard with safety much in mind. At the same time I note that Bill Aldrich’s layout (in the Realistic Layouts series on modeling the trackside scene) has some litter on the yard that author Paul Dolkos points out would normally be seen only near a cleanout track. You’ll need to balance the correctness of a safety-first yard with the visual appeal of some lading material, spilled grain, and so on. My likely audience–average age seven years old–is not going to care much about prototype practice, but they’ll think it’s cool to have some broken “glass” or a puddle or a few weeds here and there in the yard. Your audience may prove to be a tougher crowd.