There seems to be some confusion about the law on this. Without knowing more detail and assuming no marks were left, I am with @bellac11 and hope the following expansions will be helpful.
The following does not constitute ‘legal advice.’
There are various types of offence. Some offences are ‘strict liability’ offences, meaning that if you do it; you are automatically guilty as charged. There are really no defences to these offences. An example is being caught driving without insurance. Even if it’s a marginal case, you can’t go to the court and say, ‘Please Judge, it only lapsed yesterday and I renewed it as soon as I realised.’ If you’re caught driving without insurance on a public road, you’re guilty of driving without insurance.
Some offences however, you can raise a defence to. A defence laid out in s58 of the Children Act is that of ‘reasonable chastisement’. This is only a defence to Common Assault, which is the most minor form of assault. There is no defence to any of the more serious types of assault for obvious reasons.
If charged with common assault against a child therefore, one can go before the court and raise the defence of ‘reasonable chastisement’. This leaves no marks, bruises, cuts or scratches, is done with an open hand away from the face and without the use of any implement such as a belt or spoon.
There is no chastisement defence for inflicting more serious harm on a child, so assault occasioning actual bodily harm, grievous bodily harm, or wounding.
Police, teachers, social workers and lawyers get to know the thresholds and these are not necessarily where our moral thresholds lie. Cases that are charged will tend to be above the level of common assault for the reasons explained above. To those saying the OP is guilty of assault, in my personal view, it is likely based on what we know that she would succeed in a defence of reasonable chastisement. (I have already expressed my own moral views separately and elsewhere and wished the OP well.)
Hope this clarifies.