Hi Tanya,
You are considering two very fun events for little ones at
Disney's Fort Wilderness Resort & Campground. I have attended and enjoyed both of these events.
Mickey's Backyard BBQ is a high-energy evening, with a great spread of picnic food as well as music, entertainment, and the opportunity to dance with Mickey Mouse and his friends.
Chip 'n' Dale's Campfire Sing-A-Long includes singing some camp songs, enjoying a campfire, and meeting Chip and Dale, all followed by a movie under the stars. Please do note that all character appearances are subject to change.
I think that both of these activities are very well-suited for children the ages of your children. Which one you choose might come down to logistics. Mickey's Backyard BBQ is only offered on certain nights, and only seasonally. Use the link that I provided to Mickey's Backyard BBQ to check if it is offered on the dates when you are traveling, as well as if it is offered on a date that is convenient for you. Also, because it includes a meal, there is a cost associated. Mickey's Backyard BBQ starts at 5:30 p.m., which is nice and early for your little ones. It is best if you make a reservation ahead of time for Mickey's Backyard BBQ. Chip 'n' Dale's Campfire Sing-A-Long is offered nightly, you do not need to make a reservation for it, and there is no charge. It is sometimes a little later in the evening as it is scheduled so that it will be dark for the movie.
If I was making this choice, I think I would go with Chip 'n' Dale's Campfire Sing-A-Long; however, either one you choose, I am sure that you will have a great time, Tanya!