We are not being told the full story here.
The police car was coming towards you on the opposite side of the road, your daughter was in the back of your car, the police then stopped you as the child was not wearing a seatbelt , is that what happened ?
If so, the police MUST have seen that the child was not wearing a seatbelt as they came towards you or at the least as they passed you,how can you say the child only undid the belt after you were stopped? Sorry the story doesn't add up. Have you actually been charged or only warned? In any case do not even think of appealing,