Should I help this young Magpie?
It's best to leave the bird alone. The parents are likely still in the area and tending to the young, including feeding and chasing away would-be predators. You're better off staying away so as to not intimidate or upset the parents. If you have outdoor pets, relocate them indoors or to an enclosed area for a few days.

Failing this, the advice given in comments to call your local animal rescue is best.