I wasn’t there, but reliable sources say this exchange happened last night while reading Alexander and the Wind-Up Mouse at bedtime:
BOY: [LOOKS AT NEXT PICTURE] Mices. Two mices.
HRH: Almost! When there’s just one, it’s a mouse. When there are two, they’re called mice.
BOY: Two mice.
BOY: One mouse… two mice.
HRH told me that it was incredible to sit there and watch Liam recognise that there should be a collective term for several of one kind of thing, and extrapolate it from a word he’d heard us use. “Language development is so mind-blowing,” he said. “I am so the wrong person to be teaching him this.” Which isn’t true, of course, but points to how overwhelming it can be to observe a small creature learn like this.