A viking (or more precisely put, early medieval Norse) person would be considered an adult, with most of the responsibilities, at the age of 12.

Prior to that, children were raised and educated by their mothers and extended family.

Toddlers and younger children would be raised mostly by mothers, though most settlements had an extended familial bonds allowing for aunts, neighbours and grandmothers to take care of larger groups of children, while their mothers worked. We have numerous finds of toys and nicknacks from that age, including spinning tops, various game pieces, toy horses (some of them on wheels) and even toy swords. The vast majority of those toys were made of wood. (an example of a toy horse from Trondheim)

From our modern point of view, the raising of older children was pretty liberal. Older children were allowed to run around and get in mild trouble, swim, and even sail boats. Wrestling and mock-fighting was encouraged. The realities of a viking village life were such that children were not particularly shielded from difficult topics, like violence, death, sex etc., since people lived, died and had sex under the same roof.

Older children were expected to help with everyday work and learn on the job. keeping children at home, and not out-there was frowned upon, as it would prevent them from learning a trade. It was sometimes customary for older children and young adults (especially boys) to be sent to a foster family to learn a trade,and strengthen the bonds between families (sometimes as a preparation for later marriage).

Another aspect of childhood in the viking age was that due to disease (like the plague that ravaged Norway), starvation and harsh realities of medieval life, children often died young. Over 30% of children did not live to see adulthood, with at least 5% dying at childbirth. This means that a viking child who lived up to the age of 12 would have likely had witnessed death of his peers or siblings.

Simpson, J.,. Everyday Life in the Viking Age.