Hello Nicoleta
Yes, you can, but you have to stick to the schedule as indicated on the ticket. It kills any flexibility.
It is not recommended to purchase Supersaver tickets when arriving by air.
For the child, the Junior Card is strongly recommended. Also, if you will travel a few days, the Swiss Half Fare Card is strongly recommended if you do not want to spend a fortune on tickets.
Firstly, make up your plans, and as a last step, decide on tickets! And not the other way around; a mistake, many tourists do.
Kind regards,
Roland