1) Red + icon doesn't look greate. Please check Gmail Android app for an idea.
Generally design doesn't look Android specific. Can you play with existing Android apps to get a feeling about Material design.
Images, checkboxes everything would be greate for kids app, but for this app it doesn't fit the audience.
2) Challenges: Graph title fonts can be improved. And what is the bar at the bottom for?
3) Goals tab: There will be only one active goal and may be it should emphasize differently.
Also we may need to show progress here (i.e. 4 out of 6 weeks passed etc.)?
User is expected to check active goal a lot, so we could a little more statistics there: start date, progress, weight lost so far, total weight loss goal etc.?
Old goals can be just start and end dates.
4) Why are those colorful icons below tabs are for?