We do twice a year, including photos. Birthday and Christmas cards get sent with the contact and we hold them until the occasion. We have gone for avoiding birthdays, Christmas and exams too.
Fenton Works like this:
You write a letter and send to SS.
They copy it for files, vet it and pass it on to the BF.
BF do or don't bother to respond, sending reply to SS.
SS vet and copy, and send back to you.
Children can include letter or drawing, but contact is officially between APs and BPs, not the children.
(Or BF write first and you reply. I think it's better for you to write first as you are more likely to be organised than the BF).
If needed BPs can get help writing their letter from SS.
We stick to a formula for our contacts. No longer than 1 page A4 typed (we do change the font size sometimes). Same number of photos each time, taken at tourist spot or non identifiable location. has worked well for 8 years now.
NB make sure you include how BF sign themselves eg Mummy or Mummy X or just X. Similarly can cards they send be 'To my lovely Son' or just generic ones.