To be honest, I don't know as I'd attempt either the Monterosso - Vernazza or Vernazza - Corniglia trails with a toddler. Will you be carrying him/her in a back carrier, and are you a conditioned hiker that has used one of these before? As Carol said, parts of those trails are VERY steep and involve a lot of steps. They are also very heavily traveled; you need to be able to keep moving in spots where there's no room to step aside for faster hikers. (Yes, I've done both of them).
We'd covered the trekking/treno pass and. Via dell'Amore closure in one of your previous threads:
https://community.ricksteves.com/travel-forum/italy/is-cinque-terre-pass-needed-if-i-were-to-do-this-in-ct
I'll be doing it in a day....
A FULL day? Did you mean that your train from Florence isn't until 11:50 AM? If so, you won't arrive until early-mid afternoon.