This is a quick measuring activity
You need a ruler marked in both centimetres and millimetres
First, use the ruler to draw about 10 different length lines on a piece of paper.
Encourage your child to estimate the length of each line in centimetres or millimetres.
Ask them to write their estimate next to each line on the paper
Finally, give them the ruler and ask them to measure each line - and then to write the measurement next to the lines.
Repeat this a few times.
The important aspect here is that the measurements are accurate - help your child to use the ruler if necessary, and be sure they are starting their measurement from the 0 on the ruler.
You could see if your child's estimates are close to the real measurements. However, this isn't really necessary, as long as their estimates are 'reasonable'.