J.B.
I do not think it it necessary to write anything on the invite about the party schedule (i.e., when gifts will be opened). People do what they...or more like it...what their kids want to do when at a party. I have never felt obligated to do anything at a party except to have a good time :)
I had a joint party for my kids last year and we opened gifts right after having cake. Most people were sitting around eating cake. My mom helped my daughter open her gifts, while at the same time my mother in law helped my son open his. It was a little crazy, but it worked for us.