Measure the child and choose a size based on the child’s length. You can also have a look at the chest width columns if you’d like an idea of how Alice will look on your child. Alice has a loose fit, with 7-11 cm ease. If you want to size up to make the garment fit the child longer, I recommend that you only increase length, not width.
Alice was designed for knit fabrics with at least 30% stretch. If your fabric has a lot more stretch, closer to 80%, you may want to consider to size down on width.