I think the difficulty if this were to be formalised is again that the more such rules you include the more arbitrary they will be considered by those who fall outside. If you say child aged, say, under 12, people will start asking what about a 13 year old, or what about a nephew or grandchild, or what about two twins aged 2, or what about when they travel WT+, etc. In that sense, I personally really do not think that it is such a good idea to formalise those things and that sometimes, when it comes to rules, there is a beauty in simplicity.

All I can add is that even though rules say no, in your last hypothetical case of GCH+partner and small child all travelling CW and if there is room, I very much suspect that agents might decide to use discretion and let the family in GF if asked. Personally, I think that such an informal arrangement is more suited than a formal and necessarily arbitrary new rule.