Parent/Child Packages And Transaction Option Not Working As Expected

Has anyone had experience of using Parent/Child packages while enlisting them in Transactions.
I tested this on a small sample and thought that I had got it to work, but in my real-world package it does not.

The parent package essentially calls three child packages.
In each child package there are multiple DFT's that import and transform data into SQL Server.
All data must be imported or not at all.
Therefore I created a FELC container into which three Exec child package tasks were placed.
The FELC is set to Trans Option 'Required' and the Exec child package tasks to supported.
Unfortunately upon failure of one of the DFT's in the child the data was not rolled back.

So initially we had in terms of container hierarchy for the Trans Option property:
Parent package Supported
FELC for calling child packages Required
Task execute child package Supported
Child package Suppored
Tasks Suppored



Looking at this more closely we thought that we would need
Parent package Supported
FELC for calling child packages Required
Task execute child package Required
Child package Required
Tasks Suppored



for it to work.
However, the latter now gives us failures with error messages on the tasks on the child packages.
[Execute SQL Task] Error: Failed to acquire connection "Conn ECARS1CEDImport". Connection may not be configured correctly or you may not have the right permissions on this connection.

Even more strange the first couple of tasks in the child pkg complete successfully even though they use the same connection listed in the error.
These tasks also have Event handlers.