Unless the child was unusually tall or could only see while being held, thus blocking the view, I wouldn't mind having him/her move in front of me. The parent, though, would have to find the best spot nearby and should not expect me to move. If that child could only see with assistance, then again the parent should find the best spot still available and be OK with that. I don't believe that having a child with you should automatically get you the prime viewing spot.
I guess I felt that leaving my backpack, as well as my daughter's stroller, was an indication I would be back. If I had dropped those items there then left for more than 5-10 minutes, I could understand a person choosing to move my items to a less desirable viewing location assuming I wasn't coming back anytime soon. I agree with many posters that saving a deck chair, dining table, etc. that won't actually be used for 15 or more minutes is rude and selfish.
I didn't ask someone to save my place for me because we'd left the seat twice before (for the hot chocolate and cookies) and returned quickly with no problems. Also, I don't fault the woman for sitting in our place just that it would have been nice if she'd at least acknowledged that she'd usurped our place and let us back to the window while the whales were still in view.