Not all children joined the Hitler Youth. Groups of children decided to reject the Hitler Youth and create secret youth groups. One of the most notable underground groups was the Edelweiss Pirates. This group outwardly defied the Hitler Youth by attacking them and vandalizing their property. 
Their resistance shows that children followed the Hitler Youth or defied the organization out of their own volition. Therefore, while children may have been a major target of propaganda, they had a choice in following it.