My 6 year old really enjoys tangrams and has loved playing with "TanZen Lite". We have both found it much better than "My First Tangrams" which are not actual tangrams, they are just block shape pictures. While he has enjoyed playing with both, TanZen pieces are actual tangrams. They do not "snap" into place, but this is a feature that we both like since it makes it more like playing with physical tangram pieces. When completed, the puzzles pieces can have some space in between them, which is more like using the real thing. You don't know if your placement of the pieces is correct until you have completed the whole puzzle. It is a great app, and I can see it being used in a classroom.
"My First Tangrams" is fine for a block picture design, but it is not a tangram program. Also, you can put blocks on top of each other so a larger block will hide a smaller block, therefore the finished design will not look like the picture, but the program tells you it is correct. This can be confusing to younger kids. I don't know how useful this would be in the classroom.