Have the kids come over so that you know who they are. Express your opinion of them, good and bad. If you need to criticize, don’t be cruel when speaking about them. Be respectful even when criticizing. Otherwise you look like a hypocritical jerk, and you are teaching your children bad manners and possibly Lashon Hara.
Make sure that you have a good trust between yourself and kids. That way, when you make your opinion known to them – explicitly or not – they will trust and listen to you. Such trust takes years to build and starts in the toddler years.