You can use the cell and find the RadNumericTextBox inside. Note that you would need to ensure that the item is in edit mode. When the row is not being edited the RadNumericTextBox will not be rendered.
If you would like additional information on how you can access controls in the cells or rows of RadGrid you would find the following article interesting.
RadNumericTextBox numericBox = cell.Controls as RadNumericTextBox;
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