How do you approach your child when he/she doesn't like to continue at an enrolled summer lesson (in my case, piano lesson)? Our 5 y.o. does not want to continue anymore after 2 sessions only ? And she does not tell us why.

Is she the one who requested to have piano lessons this summer? Or was she consulted before enrolling? Maybe you can try talking to her in a conversational tone/mood why she doesn't want to have piano lessons anymore. Make her feel that she can tell you anything and that you will not get mad at her.