I have been in the app and gamification business for the past 6 years which is an eternity in digital years. You mention that mini games and activities do not emphasize the outcome you are hoping to achieve, and I wonder if using a fun chatbot to talk to your patients wouldn't be more interesting in this case. Children ages 8-15 love to do two things with their phones: play and chat. If you say that playing is not working then chat with them and make it playful. Speak their language, have them ask you any question they want to ask, in their own terms and language and have a chatbot ready to answer them all. You can even create a voice chatbot for the smaller kids (4-8) that still don't write very well. Talking Tom was a great success among kids because they felt somebody was actually listening to them. Let me know and we can brainstorm more about this if you're interested