In case of injury, can I have my children's friend's parents sign waivers for their children while the kids play on my property?

This question is entirely dependent upon the law of your state.  In Florida, people can release almost anything, including negligence if they are engaging in a dangerous activity (such as sky diving or scuba diving).

I can't help but wonder, however, why you find it necessary to have children's parents sign a release to play on your property.  Is there some kind of hazard on the property? 

Depending on your state law, you may or may not be able to release your duties as a landowner (which are fairly low anyway -- usually just to correct unsafe conditions that you know about or warn about them), to supervise your own children, to supervise children who are invited, or to follow state and local regulations for special conditions such as swimming pools, jungle gyms, trampolines, etc.  If you have a particular concern, I would encourage you to consult with a personal injury or real estate attorney and have that attorney draft a release.  You might also consult with your insurance agent about how your homeowner's insurance will protect you.