Drainage for sand box
You shouldn't use a tarp. It will collect water despite your drainage holes and will result in mold, mud, insects, and other grossness accumulating. Use heavy landscape fabric (heavy enough that kids toys won't shred it) and forego the rock.

Or, put the rock under the fabric to allow drainage to the perimeter of the area, and allow drainage through your walls. You might consider fabric above and below the rock to isolate it.