Hi Summer!
Don't fret! We took our youngest son to Walt Disney World for his first time when he was just shy of 6 months old! It was one of our favorite trips because his older brother, then 3, had fun exploring Magic Kingdom and being a "guide" for his little brother.
There's plenty for any child, even the older ones but young at heart, to enjoy at the parks, especially Magic Kingdom! If you look over
here, you'll notice a pdf titled "Attraction Height Requirements". The rides/attractions listed are the ones that have a requirement, and the rest of the rides at the park(s) may be experienced by even the younger ones! So if you noticed, that leaves plenty of attractions for your family to experience! The rest of the site there includes information for families traveling with little ones and it even includes a pdf of park maps with them in mind. You should keep those on hand during your trip to make navigating easier.
Hope this helps and that your family enjoys their magical vacation!!