Hi Megan! My warm welcome to planDisney, friend. Having brought our then 6-month-old and our slightly older Mouseketeer to the Parks, hopefully walking through our experience helps you!
Ultimately, it depends on the ride vehicle and Cast Members' discretion as to whether you may ride together. Our kids were usually so distracted by the magic of the ride that it was ok to split up after queueing together...mostly. I must admit there was an occasion where my toddler was not impressed to ride with Daddy. The Cast Members were calm and patient as we quickly sorted the situation. Kids happen, and I feel Cast Members have seen it all, so don't worry if plans need to be readjusted.
Rides we all shared included
Frozen Ever After (I had the baby in a sling for this as there is a drop), Living with the Land,
Gran Fiesta Tour Starring The Three Caballeros and
Journey Into Imagination With Figment at EPCOT. On
The Seas with Nemo & Friends, Cast Members politely requested that each parent sit separately with one kid. After this ride, you'll enter the fabulous aquarium within the Seas with Nemo & Friends Pavilion. Both kids and parents love this attraction!
At Magic Kingdom Park, together we rode
"it's a small world", Tomorrowland Transit Authority PeopleMover (you face each other in your own cart), Under the Sea ~ Journey of the Little Mermaid ( though I can remember this felt snug), Walt Disney’s Carousel of Progress and finally
Walt Disney World Railroad.
Another thing to note is that Walt Disney World is about so much more than rides. Our kids love the character interactions, shows and parades. Honestly, the delight of seeing one of their beloved characters in real life is just the cutest. Get the camera ready because you will want to capture all those memories. Better yet, I recommend
Memory Maker as it means Mama is in the photos rather than taking them all!
Megan, I hope this helps you and your littles have the best adventure at the Most Magical Place on Earth.
Louise