Hi Kristi,
You are right. If you are uncomfortable sending your child to someone's house whom you don't know, don't send her. We have to live with our decisions and you will never be sorry if your child is safely protected. At this point, you could simply say that you can't work it into your schedule, because you would want to know them better before you sent your daughter over. Perhaps, you could say that this time doesn't work out right now, but is there a time when the mom and her daughter could come to your house for coffee or cookies and punch and play ? that way you set the opportunity for the kids to play, and also for the mom and you to get together. it also doesn't close the door on what might be a good relationship for your daughter, and perhaps for you, too.
As your daughter gets older you will begin to send her places where you know less and less about the adults, but you do this as your trust in YOUR OWN CHILD increases. As you have faith in her ability to sense whether or not she's in a good place, whether she is gutsy enough to call home and have you come and get her should she want to be retrieved, whether she will tell you if the home isn't one in which she feels safe.
I think the best way to have playdates is to have people over at your house, moms with kids. Or to join your child on the playdate. That way BOTH parents are present, and you have some adult social time while the kids are playing.
Another option is to get involved in a church, because they have activities for the kids as well as Sunday School times. Our church has family activites where the adults AND the kids have fun together. You get to know other families, so you would then feel safe letting your children play with them.
If there's a ymca nearby, your daughter could take tumbling lessons or swim lessons or something at the Y. You will get a chance to meet some of the parents, and she will have the opportunity to meet and do things with other kids.
Honestly, the people who injure our children tend to be people whom we trust -- who else would we give access ? -- but that doesn't mean you should assume the people whom you don't trust are safe. it just means that you have to pay attention all the time, and listen to you kids so you know when something doesn't feel "right".
I think we all vacillate between being too trusting and being too protective. All we can really do as parents is to do our best, not someone else's best. and we can do only the best we can for the day at hand, with the energy and information we have at the time. So don't get down on yourself for being unsure. Just do your best, and love your kids.