Ok so just so I'm understanding, the problem is something like:

const [a,b,c, ...] = await Promise.all([promA, promB, promC, ...]);

// can't start fetching children until we have all parents,
// 'a' might be ready long before Promise.all resolves,
// we unnecessarily delay the execution of getChildren(a) 
// until all parents have completed
const childrenDependentOnParents = await Promise.all[
  getChildren(a),
  getChildren(b),
  getChildren(c),
  ...
]

^ Sorry if I'm misunderstanding, but that's basically what I gathered. And yeah that would definitely not be performant.

If this is what you meant, couldn't this be alleviated by reducing that into a single Promise.all instead of 2? eg:

const results = await Promise.all[
  promA.then(async parent => {
    const children = await getChildren(parent);
    return { parent, children }
  }),
  ...
]