Fill a jar with something countable — pasta, buttons, coins — and ask your child to estimate how many before counting.
Write the estimate down. Then count together and write the real number beside it.
Refill with a different amount and go again. Ask whether the last guess helps with this one; noticing that it does is the whole skill.
Estimation is a distinct skill from counting and a better predictor of later mathematical intuition. Recording the guess before checking makes the gap visible, and using the previous round to calibrate the next is the move that turns guessing into estimating.