'Wha gwan' (also 'whaa gwaan', 'wagwan') is Jamaican Patois for 'what's going on?' or 'what's up?' It is a friendly greeting, the equivalent of asking how someone is doing. It has spread far beyond Jamaican communities through dancehall, grime and UK youth culture. If your child greets friends with 'wha gwan' they are using multicultural London/youth English, which is now standard in many friendship groups. It is not rude, not coded, and no cause for concern. A normal 'fine, you?' style answer is expected.