I had as hard a time then understanding how a kid could think the amount of water changed, as they did understanding that it didn't change.
Maybe I can offer some anecdotal insight.
When I was five, I had a piece of cheese. I ripped it into several pieces and then analyzed my reaction to whether or not the amount of cheese had changed.
I knew that I had the same amount of cheese that I had started with. But I felt like I had more.
Why is that? I wondered. I decided that even though the amount of cheese I had was the same, it would take me longer to eat it because I would have to pick up each individual piece. This process would take longer than just picking up the one original piece and taking big bites out of it. The increased time required to eat it made it
feel like more cheese. Also, the cheese was now spread out over a larger area, again, making it
look like more cheese (in retrospect, to a five-year-old, anyway).
I was satisfied with this explanation, starting eating the cheese, and turned my attention back to
Scooby Doo.
I suspect that most kids wouldn't think about this idea on their own or can't make those distinctions at that age. It's very easy to just go with the feeling, especially for a very young child (but adults also do this).
Though...that doesn't excuse the technician in the lab where my friend did a postdoc. When she left for lunch one day, the PI was sitting with him and explaining that diluting his DNA mixture didn't give him more DNA and that he therefore couldn't use the diluted mixture to run more gels. The guy didn't get it. He was apparently fixated on part of the protocol that said
use 50 microliters and was ignoring the bit that said
of a mixture of concentration x. 
When my friend got back from lunch, they were still having the same conversation and the guy still didn't get it.
