When training as a teacher I was introduced to a non-open source product that had a reflection/symmetry tool.
When clicking on the reflection/symmetry tool the screen was divided into two by a vertical line. Anything you drew on one side of the screen was immediately reflected on the other half.
This was recommended as an introduction to reflection symmetry at Key Stage 1 (age 5-7 in the UK).
It would be very useful if Tuxpaint had a similar feature. It would be even better if there could be the option of having more than one line of symmetry for Key Stage 2 (age 7-11 in the UK). Under the Numeracy strategy children need to recognise and create images with 1 and 2 axis of symmetry.