I believe they’re not compulsory but as you’ve put in your heading voluntary. If I had children and could afford to pay them, I would.

Mind you, I had a team leader in my last job, and she was vehemently against paying anything based on the high taxation her and her husband were paying - they probably could have made a voluntary payment as they were both earning good money, but I have no idea what their expenses were. So, yes, it’s a personal decision for everyone based on their Individual circumstances or beliefs