Okay people, Chrono and crew were hurt when they fell from the cliff. When they fall, you can see the look on their faces and their legs too. Duh >.>. Also, in CC Dario falls off the cliff and died in one world... And he's like this super hard boss too..
Now about this Lavos vs Crono thing. Crono and crew neally did die, you'd need to heal them every now and then. The fact is that the Rain from the Heavens attack isn't the strong one that he uses to destroy the planet. That is a miniture version.
Also, like Gaspar said, all the hopes and dreams of all the people that Crono and crew have met is with them, making them stronger. It's back to that thing about carrying peoples hopes and dreams and making yourself strong.
Also, it's like saying "How did Lucca die?" is she was killed by Lynx, just like how he refers it to Kid by going "You'll soon see Lucca" when hes about to kill her.
They are human after all, it's not like a gun shot wouldn't kill any of them if it hit them in the heart, point blank range.
Also, about the baby falling down the mountain thing. I think it'd have a better chance of living than if an old person (adult) fell. It's like if an ant falls of a cliff, it won't die because it's to small to have impact. The smaller something when it falls from up high, the less impact. The bigger you are, the harder you'll fall