“Children” is a reference to everyone who is still learning the ways of God; it’s not talking only about little kids. Remember that St. John when writing to the congregations, called them “my little children,” but we know that he hadn’t left a bunch of little kids to fend for themselves - these were adults to whom he was writing.
“Leading them astray” means both to teach them wrongly and to set a poor example for them.
If we say that we are Catholic, then we have to show the world a Catholic lifestyle - we can’t say, “It’s nobody else’s business,” because, like it or not, people do look to us to show them what God requires - that’s why pedophile and philandering Catholic priests get so much news coverage, but pedophile or philandering non-Catholics - even teachers, scout leaders, parents, and coaches, don’t make the news, because nobody is looking to them to set the example of right behaviour - it’s not “news” when a sinner sins, but it’s front page with a special pull-out section news when a Catholic (and especially a priest, but any Catholic) sins, because Catholics are the ones who are supposed to be setting the good example.