How to tell my children why their Daddy has gone

Dear Prison Widow Blog 
I would like to reply to the post from a parent who is struggling very hard how to tell her daughter that her Daddy is a drug addict. I am in the same boat and my two children keep asking why their Daddy isn't coming home. My children are 9 and 6 years old.
I have no idea what to say at all because at their age they will not understand the first thing about drug addiction and how addiction effects people. But they still want answers as to why their Daddy has left us and I honestly do not know what to tell them. I feel very bitter right now and have cut all contact with my ex because I do not want him around whilst he is actively using drugs. It is not fair on the children or me. He had been clean for 2 years and doing so well until he was made redundant and the signs he was using again began to show. Money from our joint account went missing and was unaccounted for. I so agree that there isn't enough support for families going through this and it is painfully hard to cut all ties but needs must. There isn't even any support groups in my area I can attend or ask for advice. I am so angry that we aren't enough and the drugs once again has taken first priority! Thanks in advance. Yours Faithfully, Amanda.