You can get the child grid for a particular row using ChildrenOfType<T> extension method. For example:
var row =
var childGrid = row.ChildrenOfType<GridViewDataControl>().FirstOrDefault();
Please keep in mind that you can access the ChildGrid after it has been initially loaded which happens the first time the hierarchy is expanded and the ChildGrid is requested.
I would like to remind that RadGridView is a virtualized control and as such it recycles and reuses its visual elements on scrolling. That is why it is not recommended to work directly with the visual elements, but rather with the underlying data.
the Telerik team