The brake rubber of your Seba junior brake pad can be replaced easily. It is not necessary to buy a whole new brake pad. The brake rubber can be bought separately.
All you need to replace the Seba brake rubber is a screwdriver (cross-head).
Be careful: the junior brake rubbers are not compatible with the adult brakes and vice versa. For adult Seba brake rubbers, see https://www.slidingtiger.eu/en/accessories/brake-rubber-seba-adult-brakes.
The junior Seba brake rubbers exist in two flavours: one for the sizes EU 27-34 and one for the sizes EU 35-38. They are not compatible.