We had the exact same thing but they were also saying it about our at that time just under 3 year old daughter as well. She won't remember it they said.
Well she still talks about it now 8 months later....she even remembers Shamu's name which surprises me tbh.

As for the young one, we have pictures to prove she was there
...and she might remember this Dec or next Nov but if not the memory will be tucked in there somewhere and I'm sure she will enjoy the experience in real time.

Nothing beat the feeling of my daughter spotting the castle on the boat over and bouncing whilst singing 'castle, castle, castle' non-stop the rest of the way

Briz