Hi again Michelle,
You can take the Resort monorail at any time and it really depends on the morning that you will be going which monorail line will be faster for you to take. Once you get there just hop into the shortest line, that will probably be your best bet. You can also take the boat from TTC to Magic Kingdom (while this isn't the fastest, it is a very nice sightseeing boat ride if you have never tried that before).
It sounds like you really have some great things planned for your kids...they are sure to enjoy these special experiences. There is so much to see and do at WDW, but I have some more grown up suggestions for your 13 year old niece. One of my favorite things I did as a teenager in WDW, would be the
horseback riding in Disney's Fort Wilderness Campground. It lasts about 45 minutes and is a nice slow walking trail ride through the wooded areas of Fort Wilderness.
If she is not into horses, then maybe treating her to a nice
spa treatment day for just the girls, now that she is a teenager she may appreciate something a bit more sophisticated. She will have to be accompanied by an adult while in the Spa and during massage, water or body treatments because she is under 17, but this may be a nice girls only treat for both of you. They do have a wonderful spa at Disney's Grand Floridian Resort as well as a spa at Saratoga Springs Resort and finally, the Mandara Spa at Disney's Dolphin Resort Hotel. These are just a few things that I would do myself with my daughter at WDW that will make her feel like a young lady instead of a little girl as well as make her feel like a true princess for the day! I hope you all have some very magical moments on your upcoming trip.
Brigitta