Yes incidents like this can happen to ordinary parents I suppose, but even that would be considered lax in my opinion.
But for it to happen to a family member of the Prime Minister of the UK, (independent of what political party, that is irrelevant), needs questioning.
If the Camerons could not be bothered to check their family before they set off, what about the security personnel, who must accompany them no matter where they go?
The child could have easily been kidnapped, what then?