I can’t believe you and others are making such a categoric statement.
Firstly, it’s rather ableist. Lots of children (and adults) have sensory issues that usually won’t be diagnosed as early as 3. Badly fitting clothes are a sensory problem. I’m not talking about Charlotte but all the other 3 year olds you say won’t care. Both my children would have noticed and been unhappy for this reason; they have sensory issues because they have ADHD or autism respectively, neither of which were diagnosed until they were 12.
Secondly, a dress that was so badly fitting and too big it requires HOURS AND HOURS of adjustments sounds like it was so obviously badly fitting it would be obvious to a 3 year old. Think about it properly: a 3 year old wouldn’t be able to walk in a dress that was far too long. They’d trip up (and then probably cry).
Thirdly, in this specific instance, we’re talking about a 3 year old who may be used to fittings for formal events and is definitely used to wearing properly fitted ‘pretty’ dresses more than any of our child term. So she’s much more likely to be aware of what fits than the average 3 year old.