14yo Sole Trader Advice

Hi, my 14 year old has the opportunity to earn ongoing income as a cleaning contractor to a local business. The advice I've found online is conflicting, but my understanding is she doesn't need to register or set up a business IRD number until she earns over a certain amount. This wont be a cash/under the table situation, she will be invoicing out about 10k per annum and supplying her own consumables. Here's the part I don't understand. Does she put GST on her invoices? She will bill her work out at the end of each fortnight, but she's only paying income tax and not GST, correct? If not, can the debtor pay an invoice that doesn't have a GST component?