I'll give you some reasons:
- Child soldiers should not be punished for their crimes because they are children, and were probably forced into war.
- Children in war should be illegal because it is immoral to send under-developed people into something they probably cannot comprehend.
- The children will be scarred and suffer from illness after war.
- They are more defenseless and helpless than adult soldiers.
- They do not pose as big as a threat as adult soldiers, either for their size, intelligence, etc.
Hope this helps!