Business Rules: Get count of Children of dimension

@CHILDREN(

I am kind of new to business rules and can't find any classes to help me.  But, I have a business rule along with 2 Application Level Variables, NumChildren and ChildList.  I am trying to get a list of children in a Formaul Object.  The formaula comes out to {ChildList} = @CHILDREN(@UDA(Entity,"Boston_Plan"))  Once I add this, I save and Validate.  It comes out with Zero Errors.  I then add another Formula Object and the formula comes out to be, {NumChildren} = @COUNT(SKIPNONE,{ChildList}).  I save and validate and it says:

An error occurred in: {ChildList} = @CHILDREN(@U...,
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