Hi Chelsea,
As the mother of three children, thus making us a family of five, I can completely understand your concern about all getting to experience the rides together. I recently answered
a question about rides/attractions that have larger ride cars that can fit three or sometimes more guests. Be sure to take a look at that question that I have linked above.
In regards to a child riding solo in a separate car, this can depend on the ride and the cast member. On our last family trip last summer, my then 9 year old would sometimes be the one that rode in the next car by himself. There never seemed to be a problem with this. However, if the older two children want to ride an attraction that the youngest one can't (one with a height restriction), you can utilize the
Rider Switch service, if both of you would like to experience it. This way, the youngest can be with one of you waiting at the end of the ride (or riding another ride) while the others ride.
I hope this can help you get the most out of your trip and experience all the rides/attractions together in the best possible way. Have fun.
~Allison L