A child (with or without supervision) must also have his / her own eTA to travel in Canada. Please note, it is not possible to apply for an eTA for children that is credited in the passport of one of the parents. So every child needs his / her own passport and eTA to travel in Canada. It is of course possible for parents to apply for the eTA for their child.
Before applying, please check whether you meet the conditions of the eTA.