populate on SQL like NoSQL (with joined table)

populate.sql

SELECT 
    pt.*,
    CONCAT(
        '[',
        GROUP_CONCAT(
            JSON_OBJECT(
                'id', ct.id,
                'name', ct.name
            )
        ),
        ']'
    ) AS childs --required
FROM parentTable pt
JOIN childTable ct
    ON ct.parentTable_id = pt.id
GROUP BY pt.id --required

https://dbfiddle.uk/EzpL1aEe