The Mainsail clew (outer, lower corner) should be at least 3-6" and maybe more forward of the boom end when sail is set up normally. See picture below. In stronger winds you want to be able to have room to tighten the foot of the sail more to flatten it and "depower" or reduce the "belly" of the sail reducing the heeling power. in lighter winds you need to have some "belly" or curve to the sail to add lift and power/speed to the boat. Under these conditions, with outhaul less tight, the boom should still be level fore & aft or have a slight upward slant with the outer end higher than the mast end. Actually, as I look again at your photo, I think that sail may be OK, but you have the outhaul too tight (yes, I used to set mine that way until a wiser, more racing-oriented sailer explained how it should be setup <grin!> part of the learning curve!) Also, there should be a cleat, located further forward on the boom to tie the outhaul to so you don't need to tie it around the end of the boom. See second drawing below, if the cleat is there on your boat, it will most likely be a standard "horn cleat" (in other words, "T" shaped), rather than the jamb-type cleat I show in the drawing. (These drawings are from the Widgeon Manual that I created.)
Looks like you have quite a willing crew! I have a lot of fond memories of growing up on boats, mostly sailboats..... At around their ages I was going out with my Dad on his 16' centerboard sailboat, by the time my Sister was that age we had moved up to a 21' sailboat (with a ballasted keel), after sailing that 21' keel boat for almost 18 seasons.... it was a learning experience to sail the Widgeon, and not have that chunk of ballast to keep me upright! I never went over in the Widgeon, but did have 1 or 2 "close calls"! I learned quick to always keep the mainsheet in hand, use the camcleat to hold the tension, but always kept that mainsheet in hand, allowing me to instantly release the main and keep the boat on her feet. Sailing without the jib in higher winds helps (raise CB a bit to reduce weather-helm) as does tightening the outhaul. I added a boom-vang as well, which allowed further flattening of the sail and made downwind sailing less scary as it prevented the boom from lifting up adding more belly to the sail!
I have a lot of respect for Brian S. and his advice, but I can say that on the Widgeon. using the sail adjustments does still help on an older sail, my original Mainsail was "original" to the boat (1969) and still worked for me from 1988-91 when I finally gave in and bought a new sail. Obviously, the sail adjustments were far more effective with the new sail, but they did help (if only slightly!) on the older sail. I am still using the 1979 original sails on my DS II, should have replaced them 5-10 years ago, but financial reality has not allowed that yet. Still, tightening the outhaul and boom-vang does help enough to keep me going!
