Is second hand spoke dangerous to children?

Yes. Second hand smoking may have dangerous effects on children. It ranges from asthma, allergies, pneumonia to sids and risk chronic illnesses.
Yes, at any age. This increases chances of crib death, doubles frequency of respiratory and ear infections. Over time the survivor begins to consider smoking as a neat thing because people s/he loves do it, then another generation gets hooked.
Yes. Children should not be in second hand smoke. Asthma is triggered as well as other chronic lung conditions/diseases. This will place them at risk (if prolonged) for cancer, COPD and risk for recurrent uri's, lower respiratory infections, ear infections and general poor health. Even smok in clothing, furniture, drapes and such is unhealthy for the child.