Postnate is a relatively unknown word, but it means "a child born after another." Some synonyms for postnate include younger sibling, second-born, and subsequent child. Other possible synonyms that could be used interchangeably with postnate would be junior offspring, latter born, or younger issue. It's important to note that while these words have similar meanings, they vary in their level of formality and may not be suitable for all types of writing. It's always best to choose words that fit the context and tone of your writing, and to be mindful of your audience as well.