Wife and kids are coming but not there yet. Hmmmm. A casserole is a good idea but when are cookies ever a bad idea? If the wife and kids have not shown up, go with something kinda soft and very sweet. Fudge can be great. If you suspect the wife might show up, go with something more healthy, like maybe oatmeal. Chocolate chip might be a good compromise.
I'd definitely go over as soon as you can with an offering and apologize. I can't imagine you being anything other than up-front. Then pump them for as much information as you can about when the wife and kids arrive. Then get with the wife to really get the skinny on what is going on and why they are there. An offering of eggs might go well at this time. Hold off on the rooster until they build their coop, but maybe you could eventually offer to hatch some eggs for them.
Depending on your circumstances, an interesting welcoming gift might be a map of the area showing the names of their nearby neighbors, including kids. And what a way to get information out of them so they can be added to your map.