Hey Teri!
Sounds like you guys are looking forward to your upcoming vacation. Traveling with a large group will be great, you will have tons of memories. Make sure you use
Disney's PhotoPass to capture those group shots with all of your family members in the pictures.
Okay, most rides at the parks usually have stroller parking at the entrances so that you can securely park your stroller with a piece of mind. A Disney Cast Member usually keeps an eye on them but I would not leave any personal, valuable items in the stroller while riding. When traveling with a large group, I suggest that each person carries their own small bag- like the drawstring backpacks. That way each person can secure their own things when going on rides. Most of the rides that are fast, or go upside down have a pouch at each individual seat in which you may stow your belongings during the ride. Remember to grab them when the ride ends.
As far as what not to miss, I don't have enough room here but I'll give you a few attractions that are fun to do with a large group. They are as follows:
Twilight Zones Tower of Terror and Toy Story Mania at
Hollywood Studios,
Soarin' and Test Track at
Epcot,
Space Mountain and, Mickey;s PhilharMagic at
Magic Kingdom,
Expedition Everest, Its Tough to be a Bug and Dinosaur at
Animal Kingdom . All of your family including your 5 year old should be able to enjoy these attractions. Don't forget about
Fantasmic! at
Hollywood Studios for an evening adventure. I hope I have helped to guide you in the right direction. Have a magical family vacation!