256 is just the cost of the seat reservation and panorama car surcharge. (That is 32 pp, per direction). So you do not have a ticket,
You should not have cancelled your ticket. As you now must buy it again…,
Best however is to buy a day pass for your child at 19,- and saver day passes for the adults. See https://www.sbb.ch/en/tickets-offers/tickets/day-passes.html
Just to explain:
To make a trip on a railway you need a ticket, which is a receipt that you have paid for the right to make a trip on a route. So to travel to Bernina Route to st. Moritz and back you need a ticket for Tirano - st. Mortiz and back. That ticket allows you to make one outward, and return ticket within a day, on a train of your choice, without needing to reserve seats or do anything special beforehand. Just find your train and get on. That is what you had, but you had it refunded, so you do not have it anymore.
When you buy the ticket in advance you are often offered a day pass in stead, as bought sufficiently in advance day passes are cheaper. A Day pass allows you to freely use all public transport in Switzerland, so it serves as a ticket. I suggest you that this is what you should get for this trip
The Bernina Express has special panoramic cars attached to it. To ride in these cars you need to pay a surcharge and get a reservation. That is 32,- pp per way, and is on top of your ticket. That is what you have right now. The reservation is not compulsory, as with at ticket, but without the surcharge and reservation you can just travel in the first three cars of the train.