The optical illusion in this problem creates a good opportunity for children to conduct an opinion poll. If each of them asks ten people outside school then you might decide to pool everyone's data and create a large sample, although this is not necessary at all - it depends on your focus. You might decide to place more emphasis on how the data is displayed.
What might be more interesting is to show just the second picture to a group of people who haven't seen the first and to see whether they immediately say the squares are the same colour. Alternatively, it might be interesting to show the first picture to a group and tell them the proportion of people who'd seen both images who thought the squares were the same colour to see whether they are
influenced in any way by the statistics!
gives more information about the illusion and possible proofs.