Hi Ruth!
Your family will love
Blizzard Beach!
There is something for everyone at this water park, including family-style raft rides, areas for little ones and thrill rides for older children.
I found a great map of Blizzard Beach on an unofficial Disney site, wdwinfo dot com. You can review the map to see exactly where the Tike's Peak (children's area with slides, fountains and jets for little kids) compared to some of the thrill rides, such as Summit Plummet.
I can understand your concern about wanting to keep your kids together. My kids are also spaced in age (9 and 5) and they often have separate areas of the park that they want to explore. Often the entire family will go over to the attraction that my older child wants to ride and my husband will go on it with him while I stay with my little one. Then we go over to the smaller kid area and my son will enjoy an ice cream cone or snow cone while she plays.
The other option is to experience attractions that they can both enjoy, much as the wave pool.
You can also find a map for Typhoon Lagoon on the wdwinfo site. This will help you decide which park will better fit your needs based on the attractions you want to experience. Blizzard Beach does have more thrill rides, which are great for older children, while Typhoon Lagoon has several family attractions, such as Castaway Creek and the wave pool.
I hope you have a magical vacation!
Magically,
Stacy