Romy asks “What’s a boy?”
One reason for this might be that it’s issue of terminology rather than the non-existence of young males.
Romy may know of mermen since she’s a mermaid, and she may have learned of male fairies even though they aren’t shown, but she might not know of “boys” since she wouldn’t have learned of humans who are both young and male. Although she has learned of the king, he is an adult man and thus doesn’t qualify.
Examples of related questions may be referring to a goose and asking “I wonder if she has goslings?” or a swan and saying “I wonder if she has cygnets?”. One might ask “what are goslings?” “…cygnets?”. Though one may know of baby geese or swans, they might not know the terms for them.