Sytes,
A few things for you to think about...
I've done a fair bit of trapping, not as much as some, but more than enough to have an informed opinion about it.
For starters, you dont have to worry about your nephews leg getting snapped by a trap. I've had my fingers pinched by just about all the traps I've ever set at one time or another. Every size leghold from a number 1-4 and while not the most pleasant experience I was never fearful of "snapping" even a pinky finger...let alone a leg. Of all the animals I trapped, I rarely had anything with a broken leg in a trap. I did have a few muskrats wring off a leg...but if you've ever skinned a muskrat...you'll notice their front legs are tiny for their body size and really frail. The only traps I was ever leary of were the 330 conibears...I didnt mess around with those and I would never set them anywhere I thought people and/or their pets would come in contact with them. I never had a problem while using them.
As to posting a sign...not a good idea. I had enough trouble keeping thieves from stealing my traps in some areas without posting a sign that says, "hey, I have a string of $12-$20 a piece traps set near this sign...see if you can find them".
Lastly, I think its about time that MT and other states consider requiring trappers education, just the same as they require hunters education, bowhunters education, etc.
It makes sense to me to have experienced trappers teach others how to trap, how to do so responsibly, where to set and where NOT to set traps, etc. With trapping becoming largely a lost art, I think there are just too many new trappers that dont have a clue whats going on out there making some poor choices in trap placements. Those poor choices catch peoples pets and those type of things make the newspapers and the 10 O'clock news. They also start the unfounded and largely untrue "facts" about trapping.
When trapping is done right, by responsible trappers, its a really neat outdoor experience. I learned a lot about wildlife, how to read sign, animal habits, etc. through trapping. Most good trappers I know, are also good hunters and good woodsmen in general.
I hate to see trapping stopped in any state...its a part of our nations history and there really is an art to doing it right. I've had many, many great days in the field while running a trap line.