Hello Jennifer! So glad to have you come visit us at the Disney Parks Moms Panel! It sounds like you've got a fun group of little ones getting ready for a magical time at the Walt Disney World Resort. You are asking all of the right questions to get ready for some fun in the Disney Parks!
The Walt Disney World Resort website does provide height requirement information for each attraction in all four of the Disney Parks. The information can sometimes be a little difficult to find if you are not too familiar with the website. You can find all of the links for the height requirements for attractions right here:
Magic Kingdom Park,
Epcot,
Disney's Hollywood Studios, and
Disney's Animal Kingdom.
As far as how many Guests can be seated on individual attractions, that answer is not as straightforward. It may depend on the size of the Guest or individual safety requirements for each attraction. To enjoy
The Seas with Nemo and Friends attraction at Epcot, Guests board a "clamobile" which, based upon my experience, can easily hold three Guests and probably four on the smaller side. If you take a look at the links above, you will find video and/or photographs of each attraction. Viewing those might give you a good idea of how many Guests can be accommodated on each attraction.
Also, all four of the Disney Parks offer
Rider Switch. This great service allows parents/adults to wait with a non-rider while another parent/adult experiences the attraction with a rider. When the parent/adult rider returns, the other parent/adult can experience the attraction without waiting in line.
Jennifer, you and your little ones are going to have a great time in March. Please come back to visit us at the Disney Parks Moms Panel if we can be of further assistance with your planning.
Hakuna Matata! Jonathan G.