Wow Mr Candy’s those are some very thoughtful and considered questions with no easy answers. Collecting information from children is particularly fraught and you will need some guidance from a legal perspective. At the same time, you don’t want a system so complex you can’t serve customers in a timely manner or give them the happy ice cream buying experience you want to deliver.
This might sound like a bit of a cop out but maybe contact MBIE for guidance, or even talk to some one like Maccas or Restaurant Brands (their key contacts are on Linkedin so you could message them there) about how they plan to cope with kids in store at the same level. They are bound to have given these questions some consideration and I hope in the current environment can be generous with sharing their thinking.
Hope those tips help and well done on your thoughtful approach. Nice to hear people taking the next stage seriously.