Visa application to HK

I am a government servant of Ministry of Education and my passport is official passport. I have already received an invitation letter to attend ICMME at IMO at The HK Institute of Education which will be held on 10-13 July 2016. I will leave 8 July from Mandalay, Myanmar via Yangon international airport and be back 14 July from HK. How can I get the entry permit and where can I apply? Can I apply at Embassy of China in Mandalay? If my son ( attending the first year student in University of Computer Studies, Mandalay) who has ordinary passport will accompany with me, how can I apply the entry permit to HK for him.