I think Dublin schools use 'clann' to mean family, getting pupils/kids to use it when describing family size.
But I think this may likely be a mistake and 'clann' would be used specifically by the parent to describe the family, and not the child. When I learnt a little Irish in Galway (instead of Dublin), 'this seemed the way. It was more appropriate to use 'muintir' or 'teaghlach', to talk about family or your household.
I guess it makes sense when you consider 'the Children of Lir' - here, clann could mean children/clan/family, but most appropriately, it relates to family and specifically the offspring of parents.