Place a Diameter dimension in the drawing.  It will have the DIA. suffix.

The down side to using a Child Dimension Style is that it is actually a separate dimension style.  If you make a change to the parent dimension style, the children will be unaffected.  To avoid this use the Dimoverride method.

Radial Dimensions

Radial dimensions can be created with RAD. following the dimension text instead of the R preceding the dimension text using the same concepts.