Is it permissible to supplicate for the forgiveness and mercy of a deceased child, just as one would for an adult?

The deceased child is not prayed for forgiveness because he has no sins, but rather for mercy. As for praying for his parents, it is sunnah (definition) to pray for them that Allah makes him a [precious] fore-runner and a treasure for them, that He makes him heavy in their scales [of good deeds], magnifies their rewards through him, places him under the care of Ibrahim, and protects him by His mercy from the torment of Hell.