This issue has already been nicely dealt with by Mikhail Shereshevsky in his book Endgame Strategy, but here I would like to focus the same thing for kids or beginners. While teaching profitable exchanges to children most teachers prefer to teach them by the value of the material; for example if you are getting rook against bishop, exchange it because the rook is more valuable than a bishop.
However, I am not at all in agreement with this teaching method. The reason is that whatever is taught to kids provides a foundation where we are not focusing on the relative value of the material.
So what am I doing that is different? While teaching exchanges I always ask my student the following question for a given position. Should White take rook and pawn for bishop and knight?
Most students replied with ‘yes’ because they got 6 points against 6 points. We all know this is not correct, so the question comes how to teach relative value of the material to kid effectively and simply? I give 100% credit to the head coach of the organization who found the technique that is given in the steps manual (a manual for chess trainers, officially sanctioned by the Dutch Chess Federation). In the book examples are given for a different purpose but here we will use it for teaching child relative value of material.
With following exercises we make them understand the relative value of pieces, for example in last diagram children will play against 3 pawns with bishop and vice versa so they will realize that pawns are having more value than bishop in certain situations. Our focus is to teach kid to consider relative aspect of material rather than absolute while exchanging materials.
These kind of exercises which are fairly simple but serve an important purpose; determining the true value of material and understanding how it can be used.