Also, don't coach the kids or threaten them to behave (because you don't want to make a poor impression). Kids sometimes act up/misbehave (especially when they're "on display"), and "they" are used to seeing that - what "they" will be looking for is how you handle the situation. Clean and declutter the house/apartment, but you don't need to make it sterile - kids live there and evidence of that shouldn't harm your case at all. Don't leave dirty dishes in the sink, empty the trash before "they" arrive, some flowers are always a nice touch (as is the smell of freshly baked cookies!). Dress appropriately - not a business suit (unless you're on your way to or from work!), but not baggy & ripped up sweats, either. Like you'd dress if you were taking the kids somewhere.
Basically, be as normal as you can be, even with your stomach in knots. And do not badmouth Dad.