Practicing this skill is great fun. I always come up with a ridiculous number to make my son laugh, he loves to prove me wrong. You can use any items you have at home.
Put water beads, marbles and water in a water tray. Ask your children to hide when you fill up a small container/jar with them. Next ask your kids to guess how many did you put in. Let them record the answers and check by counting the items. You can also try to estimate how many breadsticks are in a glass, cheerios in a bowl, biscuits on a plate.