is it possible to downgrade windows 2008 enterprise Child Domain controller to 2008 standard?
No, you cannot "downgrade" Windows to a "lesser" edition.
What you need to do is stand up a new Domain Controller on Server 2008 Standard, then demote and delete the Domain Controller you want to get rid of. If you currently have two Domain Controllers for the domain in question, you do it in the other order - demote and delete the one running Enterprise first, and then install 2008 Standard and run
There are several questions here:
Can I downgrade Windows Server 2008 Enterprise to 2008?
I believe without re-installing windows you cannot just attempt to activate a non-enterprise license on that machine
Can I downgrade a child controller
Ok, I believe there is no difference in A/D between Enterprise & Standard, so downgrading is actually not going to change anything. Also, once you are a secondary controller, there is no lower to go. You can either just remove or promote to a PD.
So, you might want to rephrase your question as to what you are wanting to achieve