taya kyle , 40 remembers crying as she told her young son and daughter .
reveals the difficult moment for new upcoming abc 20/20 show .
navy seal chris kyle was shot dead in february 2013 by eddie ray routh .
routh was convicted of murder and sentenced to life without parole in february .
taya kyle will release new book in may called , ` american wife : a memoir of love , war , faith and renewal '