The repeated use of the phrase that children were "used by all sides" is concerning because there were many countries that used no children except those that got in by pretending to be older.
Iran systematically employed 12-16 year olds en masse for the purposes of clearing minefields and drawing Iraqi fire. Perhaps 50,000 Iranian children were sacrificed in this manner. Iraq regularly responded to these child-wave tactics by deploying mustard gas.
Just a few generations ago. If you were 14 in parts of europe during wwii theres a good chance you were in some army or a rag tag band of partisans. Probably a lot of 14 year olds took up arms against the US in wars since if I had to guess.