Onion seeds, also known as Nigella seeds are one of the oldest spices known to be used.
It is also known as Kalonji, and the "seed of blessing". It is considered to be one of the greatest healing herbs of all times.
The Nigella - Onion seeds are about the same size as sesame seeds but are glossy black and triangular in cross section.
The activity of learning Edible Seeds for children is made easier here.