A plastic sizer can be removed once the child grows and needs a larger size.
Balfour even has a handy ring sizer that can be printed out and wrapped around the finger to determine what size ring to buy.
In addition to their wide brims, there is UPF50+ and UV protection, a chin cord so that the hat won't slip away, an adjustable crown sizer, and in some cases, even a back flap to protect your neck.
On Spence Diamonds' website you can find a printable ring sizer and educational articles including information about diamond shapes and the 4 Cs of diamond quality.
A sizer will accurately measure a finger and determine the correct size ring.