I am not sure I understand your question completely. Maybe give me an example to clarify? However, Having a parent-child relationship between the dimensions of the source and target modules does not preclude you from using the functions SUM or LOOKUP.
Generally, the error level mismatch is caused when the source and target modules in your formula don't have the same dimensions and you didn't provide a "mapping" for that missing dimension in your SUM or LOOKUP.
An example would make it clearer to explain the issue, so please provide one.
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"