Hello Yoillauwoing97
For the Swiss part of your journey, you can buy a Junior Travelcard and your 3 children will travel for free, but you must buy this card and the tickets after your arrival in Switzerland at the ticket office at the airport. It is not recommended to buy non-flexible tickets in case of flight delay.
For the German part of the journey, e.g. from Konstanz to Freidrichshafen, you can buy tickets on bahn.de, in DB Navigator app or on vending machines in German stations. Children under 15 will also travel for free. Such tickets are not available from SBB.
For the return journey, Supersaver Tickets may be available for Switzerland if you buy them in advance.
Kind regards
Danhiel