Go over with a smile and ask them which book they're laughing at. Show them some other funny baby books and ask them which books they used to like when they were little, then ask them if they can help you find a good book to read to your Under 5s. Show the boy with the crisps where the bin is so that he can throw the packet away when he's finished.

Feedback

Well done for approaching the group with confidence and with a smile. The young people will be so surprised to be greeted positively in these circumstances that they are likely to respond well to you.
It's a great idea to find a way to involve the young people positively in the library, even if it is with the baby books. By doing this, you're showing the young people that you don't judge them for their reading choices.
You're right not to make a big deal about the crisps. Showing the young man where the bin is gives a gentle signal about respecting the library environment. Now that you've got a good relationship with the young people you should be able to ask them not to eat or use phones if it comes up again.
You've done a good job of getting their interest, now you can find other positive ways to involve them in library. Ask them if they think the baby area is more fun than the young people's area and get their ideas for improving the stock and layout of the space which is meant for them.

Reader Development FAQs for more ideas.

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Now go on to the next scenario, Understanding Young People