Would be nice (if you continue to work on this) to have a main menu/start screen to go back to when you win or lose. Maybe add some sound and do a little more with the graphics. Have your kid record a few sounds...I think it would go great with it.
Though I don't like tower defense games generally, this is pretty well made and it shows. The unit stuff works well. Commanding the units (especially when you have dozens) is pretty satisfying