Hello Agandhi
You can buy a monthly pass for the city/region of Geneva and use all public transport within this area: https://www.unireso.com/
If you are referring to the Swiss Half Fare Travelcard from Swiss Travel System, you can buy it directly at the counter in Geneva or online here: https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
You can buy these offers in advance. The Junior Card for your daughter, can only be purchased at the counter.
However, if you buy an offer from Swiss Travel Pass, your daughter will receive the Swiss Family Card and therefore does not need a Junior Card.
I wish you a pleasant stay within Switzerland.
Kind regards,
FatimaH