Problem with Global Catalog

There is a parent and child domain setup configured on my network. Rightnow there are two domain controllers configured. There is a global catalog server which is one the parent and child. Now we had disconnected the parent. There is some testing work going on. During the same I found that the first admin is able to log in the child but somehow there is a issue with global catalog server. It is not in the connection. I performed DCDiag and listed the child server. But still things are not working well.

There can be multiple reason behind the problem you are facing. I am providing you some links that will give you more information on the problem you are facing. I think the major thing which is causing problem is name resolution. But still you can refer the links below to collect more information.
How Domain Controllers Are Located in Windows XP
How Domain Controllers Are Located in Windows
How to optimize the location of a domain controller or global catalog that resides outside of a client's site

Here when a client system attempt to find a DC for authentication it collects the list of DC IP via DNS. Now the client passes a query to the DC and simply find the match for authentication. AD here grab the same thing and move that to Net Logon. It then look for client ip and then subnet-to-site mapping. If the client ip is not found in the same the DD sends back a null site value. The problem that you are facing is due to DNS. It is not responding with IP.