A swimming belt is a good choice. You can also use a swimming belt to help young children improve their buoyancy skills. As the for child's buoyancy increases. Your for child will feel more comfortable wearing a swimming belt with back buoyancy.
Adjustable Swim Back Float Belt for Young Swimmers EVA Buoyancy Support
Adjustable Swim Back Float Belt for Toddlers EVA Buoyancy Pool Beach Training
Adjustable Swim Back Float Belt for Children EVA Foam Buoyancy Pool Use
Swim Back Float Adjustable Belt with High Buoyancy EVA Material Support
Swim Back Float Belt Adjustable for Ages 2-8 EVA Buoyancy Pool Training