feat(datepicker) add child to DatePicker

datepicker/app/components/datepicker-hooks-css.jsx

@@ -18,8 +18,11 @@ const DatepickerhooksCss = () => {
 		selected={startDate}
 		onChange={(date) => setStartDate(date)}
 		onCalendarClose={handleCalendarClose}
-		onCalendarOpen={handleCalendarOpen}
-	    />
+		onCalendarOpen={handleCalendarOpen}>
+		<div style={{ color: "red" }}>
+		    Please select a day from the calendar!
+		</div>
+	    </DatePicker>
 	    current date: {startDate.toString()}
 	</div>
     );