The child FeatureCollection would be considered a Foreign Member as per the specificationspecification
So it is a legal object, but applications will not necessarily treat it as a GeoJson object.
Another approach is to use a JSON array of FeatureCollections. Note that the array is just JSON. Only the items in the array are GeoJson objects, since according to the specspec,
A GeoJSON text is a JSON text and consists of a single GeoJSON object.