If its a clean dog park, its unlikely the dogs would pick up parvo and carry it on their feet back to their home. Exposure to vaccinated dogs is actually the best way to slowly introduce your pup to certain pathogens. Now...I don't know how prevalent parvo is in your area, but it's not that high of a risk in mine (that being said, my dog did have parvo as a puppy).
As long as the relatives haven't had a dog with parvo in the last 2 years in their yard, its probably fairly safe to go into their yard with your dog.
You should ALWAYS take the opportunity to socialize your dog around nice, safe, calm, well trained, adult dogs. Your puppy will pick up a lot from them, and if they're gentle with their corrections they will teach your puppy a lot about dog/dog interaction as well. If you know these dogs, trust these dogs, then I really don't see a reason not to let your puppy play with them.