Please, kindly note, that coins which can be earned throughout the Eva's / Bruce's / Lizzy's adventure events belong to event items category. Just like all other festival items, they get reset after the adventure is finished. So no need to worry. Family adventure events will be repeated, and you will get a new opportunity to get coins for generators' upgrades!
🌵 A little tip: you can spend all the coins during the adventure, no need to wait for the generator to produce the next bonus