Just a thought: The blue tarps for covering stuff come in the 8x10 size (I think they are actually 7"10" x 9'10") why not get one or 2 of them & see if that is enough coverage for the kids before cutting your silnylon.
A cat cut do reduce that annoying flapping that straight cut tarps seem to have. If you do the cat cut, you can't (really) change it back, but if you do the straight cut, you can always re-do in a cat cut.
When you have a backpack on, no matter where you are, you’re home.
PAIN is INEVITABLE. MISERY is OPTIONAL.