I'll preface this by saying that Elasticsearch doesn't support multiple types in a single index in 6.x, and will be removing types completely in the future. Just something to be aware of.
_type is basically any other field, but it happens to have special meaning to ES (for now). it sounds like you are trying to index two different types of document,
child, in a single index. While this is certainly possible, it can result in unexpected behavior. For example, if you filter or query on a field, the results will not include any documents that don't have that field.
All that said, you can generate a table with both values, but there's no way to combine both into a single field. So you can do this:
type | fathersFirstName | firstName
------ | ------- | -----
parent | | name
child | name |
But not this:
type | name
------ | -------
parent | name
child | name