Create a parking permit scheme, like the Blue Badge for disabled people, for people with children under 12 in the car. This should create the same or similar rights and restrictions as Blue Badges, including it being an offence to park in designated space without displaying a permit.
You may be interested in these active petitions
It is very frustrating when someone without a child parks in spaces that have been set aside for people with children. This means drivers with children have to park further away and have less space to open doors to get the children out safely, without knocking into other cars. It is also less safe to have to take one or more small children on a road rather than on a path nearby the allocated parking.