We read this book a while ago where the main character - a girl - befriends a boy and they realize that something is wrong with their parents. Turns out, they were going to this tree where they would blow air (life forcetheir life-force) ininto balloons as payment. There is some kind of creature that offers deals and you have to blow your life force-force into a balloon he. He chooses specific colored balloons for each character and he chooses a yellow one for the main character.