Frame bug fix
A bit of inside-ball: under the hood, the traceable image is a custom View embedded in a LinearLayout, with the "gravity" on the LinearLayout set to center the embedded view horizontally and vertically. If Frances starts a trace by putting her finger down outside of the embedded view (i.e. inside the LinearLayout), the Android event system doesn't pass the event to the embedded view, so the trace doesn't get detected even as she moves her finger over the figure. This is perfectly understandable, correct by the UI standards, and 100% wrong for this application. I had to fish around a bit (there are a couple of options that look like they fix this issue and don't quite fit), but in the end the solution was to set the embedded view as the touch listener for the LinearLayout (i.e. embedded view implements OnTouchListener) and translate the coordinates from the LinearLayout's reference to the embedded view's reference using MotionEvent.offsetLocation.
Detect All Points
For simplicity, I'd been using only the first touch coordinate (the output of getX() and getY()) as the tracing location. However, watching Frances play the game, I noticed something about the way she holds the phone: rather than cradling the back, she usually grasps it from the side, which leaves her palm resting on the screen. Naturally, his plays havoc with the touch UI. Fortunately, the fix was simpler than the frame bug: I just read all the touch coordinates and try them all. If you put your palm on the image, that might count as a trace; I'm sort of fine with that, really.
Final Thought: On Users
Watching my niece play this game has been a great motivator for me to fix up the quirks. It's one thing to know you have thousands of anonymous users somewhere out in the world; it's quite another to have a two-year-old sitting in front of you straining her mind to try and understand a concept-breaking bug in the behavior of the program. The frame bug was driving her crazy, but she didn't want her uncle to do it for her; she was insistent on figuring it out herself. If she's going to be a trooper about it, the least I can do is clean up the last 2% of bugs in the toy.
By the time you read this, the latest version should be available on the Play Store. I didn't want to release just a bug update, so there's a couple of new figures in there too. Have fun!