You can't easily.
Appreciation of circumstances is an adult perspecitve, and children lack the mental processes to empathise with people who have less than they do.
Children are entirely egocentric, which feels like an anti-cosical character trait when observed from an adult perspective, but it is entirely their nature and part of the maturation process.
It doesn't hurt to remind rthem now and again of those less fortunate, but to make the fact that you provide a nice life for them which they apparently fail to appreciate, into something for which they are to be blamed, will only cause confusion and unhappiness.
They will learn - we all do.