I'd worry about the toddler with the plastic.

hmm... As in he might pull at it? Good point.

What is the toddler going to do to the marble table?...keep grape juice and red wine away from it and acedic things like pure lemon juice and vinegar. But crayons and pencil marks aren't a problem.

You could get some clear vinyl cling film.

My grandmother had a marble topped side table that survived her toddlers and my mother's toddlers ( me!). I have a huge piece of white marble that was used ontop of a table 120 years ago, and on top a plant stand 60 years ago, and I now use it as a giant cheese and charcuterie board. Unless you let lemon juice or lime or vinegar stand on it, marble is pretty tough stuff.
What is your toddler going to do to it?

I suppose there are different finishings on marble. Clearly the marble top you have is a different style to mine, which is not 120 years old, rather brand new with a glossy finish. So every etch mark is quite visible in the sunlight. 2 drops of yoghurt cleaned off right after dinner left etch marks. I can't imagine what spilled juice would do.
So what would a toddler do? Make more mess when eating compared to the rest of us.
I have read the threads about embracing the etching and how it develops character over time.
I just thought I'd see if anyone has any experience with plastic - I've read some plastic table covers leave a sticky residue, etc.

There are thicker vinyl sheets that simply cling With no residue.