child know that his visits are not always welcome
Q: My children are 5 and 4, and there's an only child, about 9, a few doors down. I don't trust him. He's rude, abrasive and often defiant. However, my boys love to play with him. My problem is, the kid comes outside to play when we're outside and I don't know how to say, "Look, kid, beat it," without being a complete jerk. Help!
A: "Hey, (Kid), how are you today?" … pause for answer … "We're having some family time right now, but you're welcome to stop by (specific day or time) if you'd like." Teach him now about the laws of dropping in: that he won't always be welcome, and that not being welcome doesn't mean people don't like him. Get his parents involved, too, by saying that you're fine with his stopping by,* but that sometimes you're going to say no and you don't want him or them to be surprised by that.
Also talk to your kids. If you warn them that sometimes you'll say no, then the times when you do will go a lot more smoothly. While you're at it, you can explain to them that saying no often has nothing to do with being nice or mean, but instead is about the right time and place for certain things. It will help them with the ability to give and take "no" for an answer gracefully is a fundamental life skill.
*You should say you're fine with Kid's stopping by occasionally only if you really are. Consider talking to his parents about the general idea of this older child around your children. It requires a lot of supervision from you, and the weight of that would likely be lighter if his parents were working with him on their end to make sure he's aware that younger kids have important limits that differ from his.