Dorchester dancers Isabella Robbins, Matilda Curd, Rosalind Hornbach, and Mairead and Siobhan Manning are all performing in Boston Ballet’s production of Mikko Nissinen’s ‘The Nutcracker’ this holiday season. Robbins is a Page, Siobhan is a Polichinelle, while Mairead, Curd, and Hornbach are performing the role of Urchin. The Nutcracker opened Nov. 24 at the Citizens Bank