Why are you doing this? Your kids should be able to find their own playmates. If you live on a farm I can see that kids can't get to anyone else's place without your help. The neighbor girl comes over to our house often to play with my daughter often. My son is annoyed that there are so few boys his age in walking distance (but we keep telling him to go visit the ones we know are in the neighborhood). We are lucky that neighbor girl is really outgoing as otherwise my daughter would sit at home complaining there is nobody to play with just like my son does...
7 is marginal. I seem to recall playing in my neighborhood at 7, but very few of my child's friends are out on their own at this age.
The local public school district allows children to self dismiss starting in 3rd grade, which is typically the year they have turned 8 and will turn 9 during the school year. That seems like a potentially reasonable time to allow them to go out unsupervised.
