The thing is, there really aren't any "easy" Wawayanda trails.
There is "easier" ie. maybe Iron Mountain trail,...Double Pond (when it's not under water), the new Plymouth Lane trail for the most part is not terribly technical and it has that nice bridge section in the center.
Two possible short loops for an accompanied 11 y/o might be: starting from the boat launch lot and making a left at the lake (technically, this is the Double Pond trail) and follow over the small wooden bridge, past the furnace, through the campsites and past the iron gate. You'll soon ride over a narrow boardwalk passing the northern terminus of the Red Dot trail on the right. Continue straight. Proceed for about 1/2 mile passing the entrance of the Cedar Swamp trail again on the right. In about another 1/4 mile (as I alluded to previously, some sections are very wet) on the right you'll see the left quarter panel of an old auto and some singletrack. Turn here. Nice trail of funneled rhodorendron(sp) with some nice bridge work in the center; only part that would be tough for an 11 y/o (or a rider of any age for that matter) is a short armored section with some rough rockwork just after the bridges. Make a left on Banker after Plymouth Lane and head down hill. There will be a swampy section near the end where you'll have to hike a bike a bit over a short stretch of planks. At the end of Banker with a horse stable on your right, make left on the semi-paved public road and ride for about 1/4 mile, re-entering the park proper. On your right you'll see the end of the Hoferlin trail and in only about 150 yards (now on the left), the entrance to Plymouth Lane where you began previously. Follow Double Pond back to your vehicle. I dunno, maybe 2 -3 miles in totality.
Another route might be: again from the boat launch lot, ride away from Lake Wawayanda towards the newer rest facility...just to the right of the john you'll see some narrow singletrack (Iron Mountain trail) that will have you crossing Wawayanda Rd. in a short while and a wooden kiosk. Behind the kiosk the trail starts up again and will eventually spit out by the ranger station by the parks' Warwick Turnpike entrance but there are a few non-dramatic turns that you might be wary of. Unfortunately I personally haven't used this route in years but it is still frequented by hikers and equestrians (but this time of year you'll encounter little of either). I'd suggest picking up a guide map at the ranger station upon entry (they might even have a dispenser/holder by the door if they're closed). After reaching the ranger station on your bike you can take the paved park road back down to the lake to your car or follow another road that runs pararel to a scenic babbling brook, making a right by the furnace. This latter bike route is a bit easier than the first I described; maybe 2 - 2.5 miles.