Children API exposes regions on read and write

The Children resource now includes a regions attribute on GET (index and show). Create and update (POST and PUT) requests accept regions as an array of region name strings when the agency has child regions enabled. Values are validated against the regions configured on the agency (same rules as the portal "Regions Assigned" field).

On update (PUT), regions replaces the Child's current set entirely — it is not additive. Send the full desired list; any region omitted from the array is removed. Pass an empty array to clear all regions.