This has probably been done, but starting this thread because I absolutely LOVE hearing stories where children have trumped adult intelligence with their own logic. (This along with embarrassing things kids say are my fave threads.)
I'll start with mine:
A young boy I know with ASD is very clever but also hyperactive and destructive, the only time he's really focused is when he concentrates on his 'specialist subject' - engineering. In particular ships and trains.
For Christmas someone bought him an exact model replica of the Titanic, which upon opening, the boy trod his foot through it and proudly exclaimed "That's more like it!" Cue horrified gasps from relatives but 10/10 for accuracy!
Another example, overheard in supermarket...
[Little girl pestering Mummy for sweets. Mummy ignoring her pleas.]
Girl: Mummy. MUMMY. Are you DEATH?
...That one makes me chuckle because clearly she's learnt not to substitute 'th' sounds for 'f', e.g. 'birf' meaning 'birth'... she's heard the word 'Deaf' and thought everyone else has been saying it wrong! 
I need a laugh today so please feel free to share your own! 