I haven't been investigating this. But from what I know, several child soldiers have defected from the Burmese army to the Kachin Independence Army. There was a report out about that recently. It's still happening. Ethnic armies are still using them, but I think that's on the decline. This was in the report of the special rapporteur that came out in March, so probably it is still a problem.
On May 10th, 2012. See this statement in context.