Q. How do I Harvard reference the EECERA ethical guidelines for early childhood?

Bertram, T. et al. (2015) EECERA ethical code for early childhood researchers. Available at: https://www.eecera.org/wp-content/uploads/2016/07/EECERA-Ethical-Code.pdf (Accessed: 4 April 2023). 

Example in your writing: 

The EECERA ethical code (Bertram et al, 2015) sets out eight guiding principles for early childhood researchers.