Can young children already have gum disease?

Usually not. Small children with just baby teeth normally don't get the disease--the bacterial plaque in their mouth is a little different. Some aggressive cases of gum disease can start in the teenage years, but not in young kids.
Possible. It's possible according to my pediatric dental friend, there are a lot of factors.
Yes. Gum disease is a broad term. Gingivits is a form of gum disease but is reversible. There are rare forms of gum disease that can affect children. See your dentist to confirm.