What would the difference be between "Pwy dych chi" and "Pwy wyt ti" ?
'Pwy dych chi' ('dach chi' in NW) is the formal or plural form of 'Who are you?"
'Pwy wyt ti' is the informal singular form.
Would you use the informal when talking to children? I can't imagine any other scenario one would be comfortable enough to speak informally to another who they don't yet know.
Is there a cultural thing? When would I use each?
This is explained in the course notes. In short: