fix(dfn): populate keystring children properly (#232)

Author: wpbonelli

modflow_devtools/dfn.py

@@ -460,7 +460,7 @@ def _fields() -> Fields:
                             or v["type"].startswith("record")
                         ):
                             continue
-                        fields[name] = v
+                        fields[name] = _convert_field(v)
                     return fields
 
                 var_ = Field(