From the Curly Purly Soaker pattern notes:
The important thing to keep in mind when choosing a yarn
is its elasticity. This pattern is based on ribbing, and wool
will always stretch over time. Some yarns lose their shape
faster than others, and it is best to avoid these yarns for
the waistband. I suggest you stay away from any 100%
Merino yarn. Yarns with mohair content are also to be
avoided. If you must use such yarns I suggest threading a
bit of elastic through the ribbing inside the soaker to help
the waistband hold its shape.

This soaker pattern has a lot of stretch and is designed to
fit snugly, keeping it nice and trim under clothing. When
you choose your size, you want your child to fit in the
middle to end of the size range.