After you do the patterning you'll know the effective range. All guns, even off the same assembly line with sequential serial numbers may shoot different. I've never known anyone to shoot more than 40 yds, but thats mainly because they haven't had to. I'm sure it can be done with the right choke, shell, and shooter combination. Different brands,lengths, and loads will shoot different. You may be totally surprised that a shell you never even considered may be the best, when you find it ten years down the road.You'll only know by trying.
I personally would not teach a kid to try to shoot too far but rather call them in closer and be sure of the shot. If I knew I could shoot 40 yds I would set a stake or a decoy at 30 and ask the kid to be patient and not shoot unless the bird came within that kill zone, and even closer if possible. Teach the kid to always be sure of the shot and accept the loss if the bird doesnt come close enough. They need to know that it's never a sure thing and it's not ok to try and shoot something out of range, and I would need to know that the kid is up to making the longer shot under real hunting conditions before I felt comfortable letting them take it. If the gun, choke, shell and kid combination work well at the range you choose to let them try, then stretch it a little more, and soon they will be as comfortable as you are in taking the long shot.