We came back a couple of weeks ago. We had 5 park flex tickets and definitely had to show our passports at one of the parks (can't remember which, may have been Seaworld or Busch).
Getting into the parks may take longer than you are used to, you may have to sign a sheet of paper at some parks (not sure what use that is to anyone) and also all bags have to be searched (well they prod around in them with a stick if you can call that searching).
We were not too keen on the idea of taking Passports with us everywhere we went but I bought a very small over the shoulder bag from Tog24 that was just big enough to fit a wallet, passports and park tickets in. It was the best thing I took on holiday - it was light weight and wearing it cross-ways over my shoulders, there was no way I was going to lose anything.
We were told that we should have photo ID on us at all times - driving licence will do - and you do not need to take ID for your kids.
We signed our park tickets before we left by the way - I just felt should we lose them anywhere, anyone could pick them up and put their name on them.