Are you saying that, when you open the form after having saved it with the checkbox checked, the hidden subform isn't visible until you uncheck and recheck the checkbox?
If so, you can add some JavaScript code to the initialize event of the subform, which will be triggered when the form is first opened. It can then check whether the checkbox is checked and make the field visible if it is, something like this:
if (Checkbox1.rawValue == 1) {
this.presence = "visible";
}