Any one ever tried to instal some crash bars on back sides to save exhaust or tank. On Sherco for example?

NO

Back when trials bikes came with subframes they would just bend with the exhaust. 

 

--Biff

Add weight, yep what a good idea?

Any protuberance outside the normal dimensions of the bike would be bound to catch on passing branches, rocks, knees etc such that you'd want to get rid after one outing.

Yep all of the above. I'm amazed at the falls I and the bike can have and no damage.

Cheers Taff

Seen some that need a full rollcage!!!, I know silly reply, but then again it goes with the post!!!

Any one ever tried to instal some crash bars on back sides to save exhaust or tank. On Sherco for example?

 There are covers made for the Sherco exhaust and tank.

Bark busters on handlebars yes.... For the back end... No, as it's a much bigger area to protect

The scratch and dent covers are about as good as you can do and not alter the bike in strange and or unpredictable ways.

Ok guys post may sound silly but the point is that the bike is for my son and the it is still a bigger than supposed so I thought of saving the bike for the starting mistakes.

That helps.  in that case the only option I can think of would be to have a custom one built.  I doubt you will find anything even close to what your after unless you have it fabricated...