Here is the size chart for choosing Cruz wetsuits for kids. These wetsuits consider your chest, height, and weight when determining your correct size.
If you happen to be in between two sizes, pick the size that aligns with most of your measurements. Generally, the most important measurements are weight and chest size.
A wetsuit ideally shouldn't fit too loosely. If there are numerous air pockets or 'bags' around the suit, it's too big for you.