You can pot the little ones up separately or leave them with the mother onion. If you leave them, the plant will grow and mound. I'll see if I can find a photo of the one growing in my yard in California. There is a reason its called "Pregnant" onion.
Now that you bared more of the bulb, gently dump the plant out of the pot, root ball and all, and add soil underneath it.
This is one of two clumps that grew in my yard in California. This clump was almost 3 feet wide.
This is the second plant, growing in a large bowl: