I agree that it is shocking not to thank a child for something they have made and give you. We all have had countless gifts we did not know what to do with, but kept for years because some dear child had made it at kindergarten or school.
A realistic evaluation of someone's capabilities as an artist, musician, dancer or sportsman or woman is surely only necessary if a child is considering at senior school level whether they could make a career at whatever it is they love doing. Here it is sometimes sadly necessary to tell children or their parents that they are perhaps not quite good enough to make the grade.
This should be done as kindly and tactfully as possible. I have always tried to say something along the lines of it being very hard to make the grade in the artistic professions, but that anyone who really cannot visualize doing anything else and being happy at it, should at least see if they can get into the relevant college.
All school children in my professional opinion have quite a lot of things they are good at, and grown- ups talking to children choosing senior school subjects or careers should point out all the things they are good at, and what they could do with the subjects they are best at.
Smaller children should be encouraged to do the things that interest them; most will either loose interest in the things they are not really good at, or keep them on as hobbies and enjoy them all their lives.