So, you parked in a Parent and Toddler bay and left your car, but without a toddler.
You say you had arranged to pick up your nephew and niece from your daughter who was in the shop and you argued the toss on this point with the operative, despite there being no objective evidence that this was what you were doing.
You reappeared with your nephew only, no sign of your daughter, by which time the PCN has been served and the operative had gone.
So, did you have child seats already in your car for the children or were you planning to take those in your daughter's car?
Did you look for the operative with your nephew in tow in order to explain?
May I suggest you think about what the operative saw rather than what was in your mind/you intended to do.
If there weren't child seats in your car then why should the operative take your word as fact? Why shouldn't they just have acted on what they'd seen?
I put matters this way to try and get you to see matters objectively and compose any challenge on the substantive matters in a balanced manner.