If you have predators of any kind, you WILL want that run to be securely covered. There's way too many horror stories to even risk it.
I know you have a 'sandbox' area planned for them to hang out in - but I would also suggest covering a bit of the run with a solid roof if you plan on letting them out in inclement weather. I hate keeping mine 'cooped up' when it rains and let them into their half-covered run. Mine like to stay in the covered area but I've heard of silkies at least that love to get all wet. Your call really. You don't mention (or I missed it) what area you are in - East Coast though yea?
Also, highly recommend using construction sand for the run instead of dirt. There's a good thread on here somewhere (forgive my not searching properly for it, impacted tooth and pain killers make thinking hard - woe is me) on the how's and why's - but basically, you dig down a bit, put some kind of cloth barrier and then load the thing with the sand. It drains better, and dries out the poop so you can just rake it out.
