European sizing is actually the child's height in cm. If your child is 110 cm tall you'll want a 110 size. It is always best to measure your child before deciding on the size to use. One of the great things about sewing with and tracing the European patterns is that if you have sizing needs that differ from the "norm" you can easily modify your pattern. For example if your child is tall and slim you can follow the actual or smaller sized lines but easily extend to the next sizes' length.
Here is a guideline for transferring European sizing to U.S.
All sizes listed are included, seam allowances typically need added, and they need to be traced onto another medium (pattern pieces often times overlap) Please see this blog post for my 2 cents about tracing and adding seam allowances.