The Kafka Streams Processor API allows you to define a Topology and connect sources, processors, and sinks. From reading through the code, it seems that you cannot forward a message to a downstream node unless it is explicitly connected to the upstream node (from which you are forwarding the message) as a child. Here is an example where you forward using name of downstream node rather than child index (https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorContextImpl.java#L117).
However, I've been able to connect processors and sinks to the topology without including parent names, i.e with empty vararg, using this method: https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/Topology.java#L423.
As any attempt to forward a message to those nodes will throw a StreamsException, I suggest throwing an exception if a processor or sink is added without at least one upstream node. There is a method in `InternalTopologyBuilder` that allows you to connect processors by name after you add them to the topology, but it is not part of the external Processor API.
In addition (or alternatively), I suggest making the error message for when users try to forward messages to a node that is not connected more descriptive, like this one for when a user attempts to access a state store that is not connected to the processor: https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorContextImpl.java#L75-L81