Dunny is Australian and New Zealand slang for a toilet, originally an outdoor outhouse. If your Aussie-leaning kid says they are "going to the dunny" they mean the bathroom. Not concerning, just regional vocabulary. You may also hear it as "down at the dunny" or "dunny door." It is mildly informal but not vulgar. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.