Toddlers can touch the slime, and begin to describe what it feels like (cold, soft).
They can observe the color of the slime and the color of the tools you use to shape and cut the slime.

Toddlers have developed their ‘pincer grasp’ and should be able to hold a test tube or measuring cup all by themselves (or at least with an adult's help) and pour the ingredients into the mixing bowl or cup.
When making a large batch of slime, toddlers will enjoy helping a grownup mix the slime with a spoon or other utensil.
Even younger toddlers may enjoy using tools with their slime, although they may spend more time poking holes in the slime than cutting or rolling it.