Concur with Minty. Using a GotFocus is tenuous at best since you have to be sure as to which control on the parent would get focus. Form_Activate should be a good choice but if you change windows because you had more than one form open, you would get a phantom activation. It wouldn't hurt but the _Activate routine would have to be more narrowly crafted.
Do it when the child form is closing using Me.Parent.Refresh or .Requery
The main form is in the form of a list of records
the other form is a totally separate form and contain the detail for just one record - is it connected using a linked field
So not and parent /subform in the context of a single form design