If you haven't done it yet, the right time to teach your child to pray is -- NOW. One very important responsibility of parents, to their children, is teaching the importance of prayer. No matter what one's religion is, prayer should be inculcated at an early age.
I started praying with my son when he can't even talk yet. When he was still in my womb, I'm sure he can hear my prayers. Babies and toddlers can remember a lot, their memory starts to develop even before they are born. I taught him The Lord's Prayer at the same time he was learning his ABC and 123.
Last year, my son already know how to lead the praying of the Rosary.
