It is corporal punishment, from the link below
"any act causing deliberate physical pain or discomfort to a minor child in response to some undesired behavior"
en.m.wikipedia.org
Corporal punishment is illegal in schools, but still legal in the home in the UK. Morally, however, it's shocking. Plus, if this is something that happens in that household regularly, which the evidence suggests, it seems to be ineffectual, given when the child is asked if they know what they were punished for, she says no.