OK, so you now need to submit an informal challenge to the PCN: -
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Dear <council name> parking department
Re PCN <PCN Number> dated <date of PCN> - Parked in a restricted street during prescribed hours
The alleged contravention did not occur, because I parked in order to escort my 7 year old (son/daughter), to his/her school opposite where I parked.
I therefore request that the PCN be cancelled.
Yours faithfully
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There is no need for any detailed description, because this is an informal challenge, so you will inevitable receive their usual Fob-Off letter. Post their response when you receive it. Others may suggest different reps on the same lines.
cp8759's draft is better, so use his.