‘The exclusion of children from full political status is an enigma which democratic politics should not allow.... what is at stake here is not simply the denial of citizen rights but the right to be a citizen’ (Bob Franklin, 1986, The Rights of Children, Oxford: Blackwell, p.24)
Citizenship is often defined narrowly by franchise (the right to vote in national elections). The status of ‘child’ is also associated in national and international law by reference to the age of franchise (the age at which a person achieves the right to vote). Thus there may be something contradictory in the idea that children can be citizens. Nevertheless, children often do act as if they are citizens, not least by taking on responsibilities within their families, communities and nations.
For an excellent discussion about the main issues related to children's citizenship and for a comprehensive list of references, read:
Antonella Invernizzi's and Brian Milne's Conclusion: Some Elements of An Emergent Discourse on Children’s Right to Citizenship' in: Children’s citizenship: An emergent discourse on the rights of the child? A. Invernizzi and B. Milne (guest editors), Journal of Social Sciences Special Issue No. 9: 31-42, Kamla-Raj.
The full special issue of the Journal of Social Sciences on Children's Citizenship: an emergent discourse on the rights of the child can be viewed here.