@StreamListener
public Stream<Long, Output> handleStreams(@Input KStream<Long, Parent> parentStream,
@Input KStream<Long, Child1> child1Stream,
@Input KStream<Long, Child2> child2Stream) {

KTable<Long, Parent> parentTable = convertParent(parentStream);
KTable<Long, ArrayList<Child1>> child1Table = convertChild1(parentStream);
KTable<Long, ArrayList<Child2>> child2Table = convertChild2(parentStream);

parentTable.leftJoin(child1Table, (parent, child1List) -> new Output(k, v))
.leftJoin(child2Table, (output, child2List) -> output.setChild2List(child2List)).toStream()

}