Simple, pseudo-psuedocodey demonstration of how to communicate data from "parent" entities to "child" entities

EntityHierarchy.cs

/// The goal here is to have some processing in one set of entities (the parents) whose results need to
/// be consumed by a different set of entities (the children), where it's not possible to iterate over both
/// sets simultaneously.
/// In this specific example, each parent entity has two unique child entities.
/// Different approaches would be necessary for one-to-many relationships, many-to-many relationships, etc.
///
/// NB I've never actually compiled this code; there may be minor errors due to typos or API changes.
struct ParentData : IComponentData
{
  Entity child1;
  Entity child2;
  //actual parent data goes here
}
struct ChildDataFromParent : IComponentData
{
  int dummyData; // ...or whatever
}

class ParentChildBarrier : BarrierSystem {}

[UpdateBefore(typeof(ParentChildBarrier))]
class ParentSystem
{
  // TODO: working on a way to avoid injecting this barrier but it's not released yet
  [Inject] ParentChildBarrier _barrier;
  
  struct ParentJob : IJobProcessComponentDataWithEntity<ParentData>
  {
    public EntityCommandBuffer.Concurrent CommandBuffer;
    void Execute(Entity e, int index, ref ParentData parentData)
    {
      // perform actual computations on the parent here
      
      // Write relevant results of parent computation to child entities
      CommandBuffer.SetComponent(parentData.child1, index, new ChildDataFromParent { dummyData = ...; });
      CommandBuffer.SetComponent(parentData.child2, index, new ChildDataFromParent { dummyData = ...; });
    }
  }
  
  protected override JobHandle OnUpdate(JobHandle inputDeps)
  {
    return new ParentJob
    {
      CommandBuffer = _barrer.CreateCommandBuffer().ToConcurrent();
    }.Schedule(this, inputDeps);
  }
}

[UpdateAfter(typeof(ParentChildBarrier))]
class ChildSystem
{
  // iterate over child entities, reading from ChildDataFromParent components as necessary
}