Hi Mahera,
Of what i understand, it is important to put the cap, even if it is for a young child, as it helps to prevent any risk of a fracture of the tooth or any infection.
my child got a root canal done when she was 4 and the dentist did put the cap on. there has been no problem at all. you can take a look at these links to get a better idea
http://www.dentalplans.com/dental-information/general-and-cosmetic-dentistry/root-canal-treatment-for-children
http://doctor.ndtv.com/faq/ndtv/fid/3778/Is_a_tooth_cap_necessary_after_root_canal_treatment.html
http://www.colgate.com/en/us/oc/oral-health/procedures/root-canals/article/when-is-a-root-canal-without-crown-protection-wise-1015
Mahera Muzammil Patel