In short, the answer is yes. Cabin crew are taught to be able to adapt an adult life jacket to fit a child. There are obviously different types of jacket, so the techniques will vary from one type to another.
In general, straps may be passed around the child and over the top of the jacket, with the jacket being partially deflated as necessary. The strap can also be routed through the child's legs, similar to the design of the infant life jacket, which prevents the jacket escaping over the head, or the child dropping out of it. The exact technique will depend on the type if jacket.