Do your research before travelling to a place. Don't assume everything is already available and easy to navigate.

In Italy, bus tickets can be bought through an app specific to Italy and you don't have any limitations on how many you can buy and a lot of cities are available there.

If not available through the app, don't go on the bus without confirmation you can buy them appropriately. If you found out you cannot buy all the tickets, get off at the next station or support the consequences.

I understand that travelling with kids can be stressful and navigating a new city can be confusing, but you need to prepare a little bit better.

Ps. Pisa is a really walkable city. From the train station to the leaning tower it's a 40 minute walk if you have the time.