I cannot tell you exactly how to get there (road signs, etc.) but I found maneuvering around DTD to be sooooo easy. I found everything I needed just from the signs that they had everywhere.
As for an admission fee, no there is not one. There would be for Pleaure Island, which is the middle complex of the three, if you were to go into one of the clubs (kind of like a cover charge). I think it is around $20 (someone correct me if I am wrong), but once you pay it, it is good to go into any of the clubs. If you have a UPHP, you should have the availability to go into here using one of your options, but I don't know exactly how the new passes work.
I loved DTD for the shops they had in the Marketplace area. There was a Disney at home store, a Christmas store, The Art of Disney, a glass store, just to name a few. There were some great restaurants (the Rainforest Cafe, for example) and some great coffee type shops. They have a very large
Lego store with a place for the kids to play (very life like lego there - would not miss this!) as well as a HUGE
Disney store with everything you can think that you want to get. If you are into trading pins, there is a great pin trading store there as well.
On the other side of DTD (sorry, but I am so tired and cannot remember all the areas), they have tons of eateries and a giant Virgin store and some quaint little stores that sell items that you would never think of buying otherwise. Also Cirque du Soleil is in this area.
As for if you should bring the kids, that is really up to you. If really depends if you want to go clubbing or want to take a day to relax and just shop. I found that kids really enjoyed this area, but tired very easily - think of it this way, do your kids get tired when you take them shopping when you are at home, or are they good for the time you are gone?
Here is a good site on what is at DTD:
Downtown Disney
I really enjoyed DTD, moreso than I thought I would. I cannot wait to go back and add another picture to my collection that I bought at The Art of Disney, or add another ornament to my tree. I Loved DTD!
Jo