I already answered your support thread and for your convenience I pasted my reply below:
I see from your code that you use a RadGrid, not a Repeater control and as far as I understand you experience problems in accessing the cells of the child, details table.
If I am right I suggest to access the cells as explained here
, because your code will not work as expected, due to having a different DataSource for the child table.
In case I have not understand you correctly or you experience problems, please send a sample, fully runnable project along with a detailed explanation of the desired behavior.
the Telerik team