Hello, Katherine. I am deeply, deeply sorry for your family's loss.
Children 7-years-old and older may ride attractions alone, provided they meet the height requirements and they are comfortable riding alone. Children under 7-years-old must be accompanied by another rider at least 14-years-old. If you are able, you may want to consider bringing along a friend or relative so you are able to ride the 'big kid' rides with your older children while someone else stays back with your little one. So everyone can join in on the fun, you can use Rider Switch to allow the other adult to accompany the children for a second ride with a reduced wait time.
If you do visit with the four of you, there are many rides you will be able to still enjoy together. The ride vehicles for attractions such as The Seas with Nemo and Friends, Haunted Mansion, Under the Sea - Journey of the Little Mermaid, and Peter Pan's Flight should accommodate an adult and 2 children. Your oldest can ride in the vehicle beside you if there isn't quite enough room for you all in one. Other attractions have bench type seating such as Pirates of the Caribbean, it's a small world, Kilimanjaro Safaris, Gran Fiesta Tour Starring The Three Caballeros, Living with the Land, Jungle Cruise, and Mad Tea Party which will allow all four of you to sit next to one another. There are also many shows your children will be able enjoy sitting right next to you, which is fantastic because you won't want to miss the looks of joy and amazement on their faces!
Wishing you and your family a magical vacation, Katherine!
~Rachel