Do you want output like this?
ALTER PROCEDURE [dbo].[spAGEwisechldREPORT]
@DISTID varchar(250)
AS
BEGIN
SET NOCOUNT ON;
DECLARE @ChildTblName AS VARCHAR (125)
DECLARE @selectQuery AS VARCHAR (max)
SET @ChildTblName = @DISTID + 'Child_D'
SET @selectQuery = 'SELECT COUNT(age) AS Count from '+QuoteName(@ChildTblName)+' where (age>=6 and age<=10)'
SET @selectQuery = @selectQuery + ' UNION ALL select COUNT(age) AS Count from '+ QuoteName(@ChildTblName)+' where ISDATE(dbo)=1 and (age>=11 and age<=14)'
SET @selectQuery = @selectQuery + ' UNION ALL SELECT COUNT(age) AS Count from '+QuoteName(@ChildTblName)+' where ISDATE(dbo)=1 and (age>=6 and age<=14)'
EXEC (@selectQuery)
END
Go