Hi Jenn!
Thanks for your question.
The one thing
Disney applies considerable interest in besides entertaining its guests is ensuring guest safety. I can't recall a single attraction that doesn't require Disney guests to be firmly seated in their seats with his or her own seat belt, a safety bar, or a shoulder harness pulled over its guests for protection. In the spirit of safety, it's unlikely there are any rides where your child can sit on your lap, and certainly not 2 seat rides.
When my daughter was 4 years old, and before the arrival of our son, we had to split ride times on 2 seat rides with our daughter. So, for example, I may ride with my daughter on Astro Orbiter at
Magic Kingdom, while my wife would later ride with her on Dumbo the Flying Elephant.
Hope this helps some. May you and your family have the most amazing time at
Disney!
Terry