While there is no specific indication for kids size range or size scaling, there is a way to make your own.
The most simple solution is to edit your size spec or block
Enter into your Product Size Specification area
If you're working on a block, enter the table view for your block from within the block library.
Click on the 3 dot icon found at the top right
Click Edit Size Spec
Toggle the Size Scale Format to Alphanumeric
Input your sizes into the space below accordingly. (i.e. 6-12 months, 3T, 4T, etc) and hit enter.
When you're done you can reorganize or rename them as you please.