If you want to append a percent symbol to the end of a numeric quantity, you have to convert the numeric quantity to be of char or varchar type.
Usually this is not advisable to do this type of thing in the database. It is much better to do it in the presentation layer such as reporting services or a GUI where you are consuming the data if you have one.
If you absolutely must do it in SQL, then convert it to varchar using cast, convert or:
STR( 100.0* SUM(TotalBaby)/SUM(TotalParent) ,10,2)+'%'