You have two choices:

1), make sure you move the focus to the subform *control*..and then execute
the move command

eg:

  Me.child1form.SetFocus
  DoCmd.GoToRecord acActiveDataObject, , acNext

The above does work quite well, but my spider sense does NOT like having to
use a focus switch.

2)

You can also put some code in the subform that moves to the next record such
as:

Public Function MoveNext()

  On Error Resume Next
  Me.Recordset.MoveNext

End Function

The above code would be placed in the SUB-FORM.

Then, in the main (parent form), you can reference the code/function like:

me.child1Form.Form.MoveNext

Note how you don't have to make a focus change. So, any public function
actually becomes a method of the form. And, of course, you can also declare
the function as a sub, and call it

Call me.Child1Form.Form.NameOfSub

So, you can build some custom public functions/subs...and call them..

if you don't want to write additional code in the sub-form code module, then
the first example of using the setfocus should work...