Estimating is a really difficult math concept for very young children to grasp. It's a fun idea to play with though, and gets your child thinking about how much and how many.
Try playing with pennies, or anything else you have plenty of. Invite your child to make a few piles of pennies and then together guess which pile has the most pennies. Count the pennies in each pile to see if you were right.
While talking you can use words like "more," "less," a few," and "some," which are great math vocabulary words.