Hi Sherri,
Both of your breakfast options are wonderful. Since you mention that you have five hearty-eating boys, you should know that each of these restaurants offers breakfast buffets, so your boys should be able to fill up at either one.
Chef Mickey's is located at Disney's Contemporary Resort. The fare for breakfast is typical American breakfast food. At Chef Mickey's you are likely to meet Mickey Mouse and his friends dressed up in their chef gear.
Tusker House Restaurant is located in Disney's Animal Kingdom Park. At this breakfast, you can find both African and American inspired cuisine. Breakfast at Tusker House is Donald's Dining Safari, so you can expect to see Donald Duck, Daisy Duck, and others in their safari gear. Please note that because Tusker House Restaurant is inside of Disney's Animal Kingdom Park, you will need valid theme park admission to dine there. Also note that characters at all restaurants are subject to change.
If I was choosing between Chef Mickey's and Tusker House Restaurant, I would choose Tusker House Restaurant. I love that I can try some African-inspired cuisine, while my daughter still had the option of more traditional breakfast fare.
As for Rainforest Cafe, many people do enjoy a meal there. My family tends not to frequent the Rainforest Cafe because it is a chain. We prefer to to eat at restaurants only found in Walt Disney World when we are there. That said, my daughter really did enjoy the dinosaur-shaped chicken nuggets she had at the Rainforest Cafe once!
I hope this helps with your dining decisions, Sherri. Please do not hesitate to ask if you have any further questions while planning your first trip to Walt Disney World!