How to setup transformation rule to map if an item is a child member of a (another) dimension parent

If the number of such departments is low, the easiest approach is to have a number of Composite mappings that map A#yourAcc:U1#yourChildDept to a specific account. 

If that's not feasible, you will have to get dirty with a Complex Expression or Parser rule. You'll probably need args.GetSource or args.GetTarget to retrieve the UD member name, and BRApi.Finance.Members.IsChild (or .IsDescendant) to check if it's a child of your parent.

If you want to take the api route, I’d recommend you have your rule getting all children of that parent the first time the rule is executed and save results in a collection  (you can achieve this using globals and storing all children in hashset).

then you just have to check if member is contained in the hashset.

in this way you avoid multiple api calls (one for each line matching the rule criteria and not mapped yet)

HTH

Thank you for the responses! I was able to resolve the issue with an IF statement:

If BRApi.Finance.Members.IsDescendant(si, _
    BRApi.Finance.Dim.GetDimPk(si, "DIMENSION_NAME"), _
    BRApi.Finance.Members.GetMemberId(si, dimTypeID.UD3, "PARENT_MEMBER"), _
    BRApi.Finance.Members.GetMemberId(si, dimTypeID.UD3, DEPARTMENT_FOR_DATA_RECORD)) _
Then

That being said, it is definitely more efficient to use a global variable as franciscoamores said.