FamilyMemberHistory.estimatedAge shouldn't be a "child" of the existing FamiliyMemberHistory.age because age is a choice of types. (Per http://hl7-fhir.github.io/formats.html#choice, elements that have a choice of data type cannot repeat. I.e. They must have a maximum cardinality of 1. When constructing an instance of an element with a choice of types, the authoring system must create a single element with a data type chosen from among the list of permitted data types. ) FamilyMemberHistory should continue to support both an estimatedAge (boolean) in conjunction with the age (estimated or actual – expressed as either an Age, Range, or string). (In the future, we can consider adding a BackboneElement with estimatedAge and age [x] elements if/when the added complexity is warranted).