I don't understand why you can't just attach a child order to any order in TWS. Currently you can only attach some auto stop or bracket order to an existing order. After you have done that, the "key field" indicates that the orders are related. I would expect that, when inserting a random order in TWS, one could use this key field as a parameter to couple that order to any other (existing) order. But obviously that's not possible. Any ideas on how to solve this? Or why it's not possible?
Note that there is a practical work-arond for this issue by adding a condition in the condition field of a (child)order. But this only works when you don't have multiple orders open on the same equity class. Example: you can make a child order with the condition that it is only transmitted when (for instance) an INTC option trades. But you can not specify what INTC option or which order of a specific INTC option (in case there are more orders of that option). So I think it would be quite useful to make the key field available in every order so you could link them.