I don't know how to share my feelings with my friend regarding my child going to her house to play.
I have an 8-year-old and when he goes over to play with his friend, I am always concerned because the mother is very free with her children. I am not comfortable with my son going to the playground which is far from her house or going bike riding around the block. I have told her this several times and she doesn't seem to get it. Now I am uncomfortable every time he is asked to go over and play. How should I deal with this?
I had a similar situation with one of our family's friends. The parents of the children let their kids roam the streets of two suburban neighborhoods in the SF/Bay Area -- something I did not feel comfortable allowing our child to do. I also didn't feel comfortable with allowing their older daughter (who was under 10 at the time) be the "guardian" of the children as they roamed the streets.
I spoke to the parents about my concerns, but they thought I was being too paranoid or over-protective. There were also cultural factors involved: the mother of the family was originally from a small village in Bali, Indonesia, where she (and her American husband) felt it was safe for children to roam free without adult supervision. They were raising their children in the U.S. with the same perspective -- that it was still safe enough on the streets of their U.S. neighborhoods, and that their children would only learn to be 'street smart' by having the opportunity or freedom to develop their independence without adult supervision.
Ultimately, I decided that we would remain friends with their family, but that I would not allow my daughter to be in their care at their house alone (i.e., without me or my husband present). Occasionally, I did ask the wife to babysit our daughter, but only at our house, and with clear instructions that our daughter was not to be left alone, either in the house or outside in the neighborhood.
I explained to our friends that we had differing opinions on this parenting matter, and that for our daughter, the rule was she was not to be roaming the streets without adult supervision. I also made sure that my daughter understood the rule.
You may need to establish a similar situation: where play dates between your son and his friend can occur, but only at your house where the children will be supervised. Or, you may need to explain to your son that he is not allowed to go to the playground by himself, and go bike riding when visiting that friend's house (following up with the mother to explain that there are certain guidelines for your son that you'd like her to respect while he is in her care). Don't agree to anything that makes you uncomfortable or too worrisome: your responsibility is to your son first and foremost.
You definately have reason to worry as so many dangers could happen. I would explain to your friend that you were raised to watch your children close. Arrange a play time where you can be there to watch the children. It will give you peace of mind. They are really too young to be left unattended.