Luckily for Gilbert, in order for his face to express different emotions, he has to make relatively large facial movements. Small changes are hard to execute with his almost nonexistent facial muscles, and being false means his reactions are always a bit slow.
This way, Hill can ensure that even a slight smile doesn't appear on his face...though he doesn't really care if he's caught or not; it's just better to do it later.
There's nothing he can do; children's reactions are just more adorable: upon hearing Hill's words, Russell's instantly focused and then floating gaze clearly indicated he was silently cursing and then realizing where he was, subsequently trying hard to control his emotions.
Not to mention Carpenter, who, upon hearing Hill's words, immediately buried her face behind her brother's back, clearly shivering.
