OK, I got it.
that's the final code:
protected void fvCurrentLoan_DataBound( object sender, EventArgs e )
{
Button btnPayOff = ( Button )fvCurrentLoan.Controls[ 0 ].Controls[ 1 ].Controls[ 0 ].Controls[ 1 ].FindControl( "btnPayOff" );
btnPayOff.Attributes.Add( "OnClick", "return confirm('" + Message.ConfirmPayOffLoan + "');" );
}
For curious minds, the first Controls[0] is a ChildTable, the second Controls[1] is FormViewRow, the third Controls[0] is a TableCell, and the last Controls[1] is an HtmlTable.
Ehehe, tricky one.
Thank you for your help, I'm closing this question.