The majority of pediatric malignancies are either germline related, in utero (de novo mutation/SV, potentially caused or facilitated by maternal environmental exposures), or a reverse lottery winner. There simply isn't enough time for somatic mutation to cause the sort of devastating fallout that you see in DIPG or infant leukemias. (Furthermore, even the point mutations seen in pediatric cases are characteristic and rare or absent in adults; some structural variants are also observed in adults, but they are much rarer and accompanied by fewer cooperating events)