Because clann is a singular noun. There is no form that means 'child'. It refers to children as a group. Often the best translation is 'family', provided it is clear that this refers to your children.
The English word clan comes from it. Although the meaning has changed a bit you can still see that it is a singular noun referring to a group of people.
It is in fact an uncountable noun, so it does not even have a plural.
It comes from Brythonic - the Welsh-like language we spoke before the Gaels arrived. The Welsh is plant, which also has a plural meaning with no plural ending (but Welsh rules on singular/plural are weird). The change from p to c is quite normal for words borrowed from Brythonic to Gaelic. The Brythonic word comes from the Latin planta. We may use the word plant for a plant of any age, but in Latin it was used for an 'offshoot' or 'young plant'. It then started being used collectively for 'all your offshoots/offspring' in Brythonic, i.e. your family. Its plural meaning is probably something to do with the weird rules in Welsh for singular/plural - for example moch in Welsh means 'pigs' even though muc in Gaelic means 'pig'. The feminine gender of clann goes all the way back to Latin planta - speakers of many languages will recognise -a as a feminine ending. D