6 is a great age to experience the magic for the first time! My niece was 6 when I took her last year - here are my tips:
Take a mid-day break for a swim & a nap. It is hot & humid and that really zaps your energy. The first day was "I'm too old for a nap!" then after that it was "I'll just take a quick rest." Believe me, I needed the nap as well! At this time the parks are open later, so some of the parades & fireworks are later and you'll want to be feeling up to returning for them. Get your hand stamped whether or not you think you'll be returning.
Drink lots of water - and take a bathroom break whenever you see one. There's nothing worse than waiting in line only to hear just before you get on a ride that your little one has to go!
Bring along prepackaged snacks. Also, a fanny pack for your DD might be handy, especially to carry her autograph book and a large barreled pen. (The thicker pens are easier for the characters to grip.) I printed up labels before we left with clip art pics of various characters and the name printed beside them; this way it was easy for her to recognize the autograph later.
Stay onsite if you can, but offsite I'd recommend a DTD hotel or Holiday Inn Family Suites. Get rain slickers from
WalMart or Target - these come in handy cause there's usually some rain at that time of year (it's so warm the rain won't be a real bother, but you will need the slickers - even sometimes just to sit on for a parade.) Another good buy at those stores would be the neon necklaces or small light up toys - get them before you go and save your money for something better at WDW - like a character meal. We loved 1900 Park Faire with Cinderella & Prince Charming, but Crystal Palace is a lot of fun too.
If you can work it into your budget the Family Magic Tour is so much fun. Siobhan loved working out the clues and still tells everyone how she "saved the world for Mickey Mouse." She & her cousin truly believe this and that is such a magic experience - for me as well as them.
Don't miss Minnie's house at MK, Voyage of the Little Mermaid & Fantasmic at MGM, Honey I Shrunk the Audience & the Kidcot stops at Epcot. Don't try to do everything, remember to relax and enjoy each magical moment. Have a wonderful time!