First, excepting an exceedingly cynical minority, leftists use this rhetoric because they really, truly believe it. They truly believe we don’t care about kids. They truly believe we conservatives want people — especially minorities — to live in poverty if it means preserving our perceived wealth and privilege. In part they believe this because they tend to live in more concentrated monocultures than conservatives, and are more used to talking about us than talking to us.
Second, as intensely as they believe we are evil, they believe in their own ideological virtue. Thus, they often take a critique of their ideas in the same way that others take personal insults — as direct frontal assaults on their character. This makes civil disagreement difficult and causes dialogue to degenerate quickly to an exercise in public shaming.