Because kids’ skin is very different from adults’ skin and much more vulnerable. As it does not produce sebum, it is badly protected against dehydration, temperature changes, pollution, and UV rays. Have a look at «Dermatological Research»
Because kids do not have the same needs as adults. To take daily care of their skin, they need to find it fun!
Finally, because when kids are in primary school, they easily integrate advices in their daily routine. Kids between 7 and 12 are constantly learning. Therefore, it is an ideal moment to teach them how to take care of their skin. It is crucial that they know how to do this before puberty.