If someone underage initiates a sexual conversation with you- which isn’t just them asking for advice or sex ed -it is up to you, the adult, to shut the conversation down. You do not reciprocate and start speaking sexually/flirting back at them, even if the minor claims they don’t mind.
You shut down the conversation and leave because you’re the fucking adult. Stop being sexual to children even if they start it and say they aren’t bothered- they’re still underage and it is your responsibility as the adult to stop it.