I use this with good results
[Forms]![frmParent]![frmSubform2].Requery
If you have a parent form with multiple sub-forms use a separate line for each form to be re-queried, e.g.
[Forms]![frmParent].Requery
[Forms]![frmParent]![frmSubform1].Requery
[Forms]![frmParent]![frmSubform2].Requery