Yep, an age old problem. We were at a fence side table in Mexico for Illuminations one time. A family came and crushed themselves between the table and the fence blocking our view! You HAD to see it to believe it!
"Pardon me. Down in front." Or "Pardon me, I'm not able to see through you, " are about the limit of my comments though. Takes from the whole experience to get awkward over a parade view.