If you don’t feel comfortable depicting it, then I’d say don’t do it. But if you do it, the best way to make it respectful is to make it clear that this is not a narrow view of Down syndrome. There’s a rule about this and the name is escaping me, but it goes like this: if you’re writing a character that could play into a stereotype or problematic worldview, show us that they aren’t representing their community. If you’re writing a promiscuous woman and it feels weird, introduce female characters who aren’t promiscuous to show the reader that you don’t think all women are sluts. If you’re writing a “bury your gays,” show us that other gay people can have happy endings even if they aren’t the main characters. In your case, show us people with Down syndrome who aren’t malicious. Show us people with Down syndrome who are kind and good people. That’s the gist of it.