Cascading deletes are not possible through a single GraphQL mutation but could be achieved through a DQL query for for your use-case.
Query:
{
relations as me(func: eq(dgraph.type, "Relationship")) @filter(eq(name, "Child")) {
rm as meta
}
}
DeleteJSON:
[{
"uid": "uid(relations)"
},
{
"uid": "uid(rm)"
}]