So you're looking for the 'cute' factor as you decide who to include in your wedding party. Your choice, flower girl and ring bearer or maybe your favorite pet carrying your rings down the aisle. Very high cute factor but is it worth the insanity factor? Be sure when you choose either a child or pet that they are well trained, I know, you're laughing, but both kids and pets need to be old enough and mature enough to follow directions and not to freak out as they walk down the aisle. You don't want pets or children likely to get so excited they, shall we say, leak as they walk down the aisle. Consider assigning a parent or one of your bridal party the child or pet is used to as an 'aisle guide.' This way your wedding will go off if not without a hitch, at least with minimal hitches!
Remember the old actor's adage, never work with children or pets? This is because it's so easy to be upstaged in the middle of an important scene. Think about if you want your pet or flower girl/ring bearer to steal your limelight. I know, it sounds selfish, but it should be your day. Bad enough to be upstaged by a child but if your Labrador Retriever steals the show (or runs off with your rings) ...well you get the picture.