Hi Gail,
Thanks for visiting the Disney Parks Moms Panel with your question. You are doing some of my favorite things on your stay at Walt Disney World! I do love a a stay at Bay Lake Tower at Disney's Contemporary Resort. The theme is so sleek and modern, and the ability to walk to Magic Kingdom is fantastic. I also really enjoy
Chip 'N Dale's Campfire Sing-A-Long. I believe that it is truly one of the hidden gems at Walt Disney World. My family and I attended this event numerous times on our last trip.
I think that you will find this event to be quite appropriate for a 3- and 5-year-old. The songs are fun, the atmosphere is laid-back and camp-like. You can use the link I embedded above to choose the date that you are planning to go to find the time it will be held on the date you want to go. I also do suggest that you double check the time with a cast member when you arrive. You should know that typically, the campfire is lit about 30 minutes before the Sing-A-Long. If you like, you can even buy the makings for s'mores right there! The Sing-A-Long itself is about 30-40 minutes, and during this time Chip and Dale will come around to meet guests and take photos. After this, there is generally a movie under the stars.
Using Disney Transportation, the best way to go would probably be to take the boat transportation from Disney's Contemporary Resort to Disney's Fort Wilderness Resort & Campground. Then, you would need to take a bus from the Settlement Area, where the boat docks, to the Meadow, where you will find Chip 'N Dale's Campfire Sing-A-Long.
I hope these thoughts are helpful for you, Gail! Have a wonderful time planing your vacation, and please do not hesitate to ask if you have any further questions!