It's really good that you've told Kelly you'd love to talk to her, but will she believe you? Young people, just like adults, can spot insincerity at a hundred paces. Even if you genuinely want to talk to Kelly, she may not see it that way.
It would have been good to at least acknowledge Kelly's reading and to suggest a time when you can talk.
Worst of all here though, is the message that you are too busy because you're serving the adults. Young people often face situations where they feel less important that adults and this is a prime example. Kelly might not bother to approach you in future. Young people deserve the same level of respect as adults. Sadly this isn't always the case and there are instances even in libraries where adults get first service in a queue even if children have been waiting before them.