Repeated contact with families after their baptism service helps to build relationships and encourages them to remain connected to their church. Send a baptism anniversary card with a warm greeting and an invitation to return to church for a celebration.
A special prayer for them and tea/cake or biscuits afterwards is all that's needed to make them feel loved and welcomed by the church family, and more likely to come back again another day.
This set of cards uses 'baptism' not 'christening' terminology, and imagery is more traditional than contemporary, so you can make that choice where appropriate. You can add contact details to the card before you order if you wish.