The credit is based on the child's age as of 12/31/21 ... there is no 1/2 year calculations. So if the child will turn 6 anytime in 2021 the credit for them is only $3000. Thus the advance credit is 1/2 of the $3000 = $1500 divided by hopefully 6 months = $250
@Critter-3, is right on the money here. Below is a link that will provide some more information re the credit.
Hope this helps.