Complex sentence: Tom takes charge and the kids come to realize the alien mind control device only works on grownups, giving them a fighting chance against the invaders, and the responsibility to protect the adults by keeping the aliens' existence a secret.
Split into simpler sentences: Tom takes charge and the kids discover the mind control device only works on grownups, giving them a fighting chance. They realize the have a responsibility to protect the adults by keeping the aliens' existence a secret.