Getting your child to wear a helmet is only half the battle. Getting them to wear it right is a whole other story. If you are not certain whether or not your child's helmet fits, ask their doctor.
Most pediatricians can tell you whether a bicycle helmet fits your child properly, or whether it is adjusted properly so why not bring it with you to your next well-check. Have your child put their helmet on during their check-up and ask your doctor to assess its fit. You'll get good advice and hearing a doctor stress its importance could potentially make an impact on your child.