Welcome aboard, Debra! Isn't it fun to begin planning a cruise with Disney? There are so many options available for everyone that you're all sure to have the most magical time!
It sounds to me like the particular cruise date you're considering may have already sold out the inside staterooms that sleep four. Some of the inside staterooms do sleep four, however some only sleep three. If the
online reservation site isn't giving you access to the inside staterooms when you select two children, but it is giving you that option when you select one child, I think that must be the case. To confirm, I'd suggest you give the agents at Disney Cruise Line a call at 800-951-3532. They can pull up that sail date and the specifics of stateroom availability for you, as well as help you book your cruise once you find the stateroom that will work best for your family.
Good luck planning your very first cruise with Captain Mickey and his crew, Debra! Stop back and let me know if you have other questions as you plan! I'm happy to help!