Hi Laura,
How exciting to be planning your first trip to Walt Disney World! I also love traveling with a larger group like you are. You can split up into smaller groups when it makes sense. I can certainly imagine a time when one adult might want to take the two-year-old back to the room for a nap while everyone else continues to enjoy a theme park.
The good news is that for purposes of booking rooms, you can consider each family a group of four. Because the first family has a child who is two years old, that child is considered an infant (until he turns three years old) by Disney. Rooms at Walt Disney World resorts can accommodate at least four guests plus an infant, if not more, depending on the resort. So anytime you see that a room can sleep four, add "plus an infant" in your mind.
The most cost-effective tier of resorts for you to consider is the
Walt Disney World Value Resorts. These include Disney's Pop Century Resort, Disney's All-Star Music, Movies, and Sports Resorts, and Disney's Art of Animation Resort. For the configuration of families that you are traveling with, if budget is most important, I would suggest booking two standard rooms. Both Disney's Art of Animation Resort as well as Disney's All-Star Music Resort have family suites, which are wonderful, but can sleep up to six. Your traveling party would need two of these and they would be more expensive.
When you make your reservations, you will need to make two separate reservations, one for each family. You can then call Disney Reservations at 407-W-DISNEY (407-934-7639) and ask for the reservations to be linked. You can request that the rooms be near to one another. Although this request is not a guarantee, Disney will do their best to honor the request.
I hope that this information is helpful for you, Laura! Please do not hesitate to ask if you have any further questions.