Yes Vicente, you are right, amigos would be better for this type of note. amistades is not wrong AT ALL but considering this is a note for a 10 year old, amigos sounds better.
Perhaps my consideration is too regional but I am also thinking about my own son and he would not use the word amistades, he would say amigos and of course, we (his parents) referring to his friends, would say amigos as well. Perhaps when he grows older...
What do you think Dragona?