What is the basis for saying that Buddha's heart does not increase or decrease?

No increase or decrease means no increase or decrease, that is, there will be no change.

This is the true state of Buddha's heart. There is a saying in the heart sutra: it is the empty phase of all laws, which is neither born nor destroyed, nor polluted, nor increased or decreased. These three words can be used to describe the state of Buddha's heart.

The Buddhist scriptures were said by the Buddha, who told us his feelings and wisdom. This state of Buddha's heart can only be felt in the realm of Buddha. Therefore, we must personally witness the realm of Buddha, in order to realize the essence of undiminished Buddha's heart.

Children are not the hearts of Buddhists, but the hearts of ordinary people who are still in infancy, so children have no compassion. Most people have long forgotten compassion because of selfishness when they grow up.