There are words for relationships obtained by marriage or partnership of family members: xxx-in-law. But, surprisingly, there is no word (that I know of) for the parents of your child's partner. Or is there? Or did we forget it?
I wish there was a word for it (also in Dutch, my native language), because it is always hard to describe this common relationship concisely.
For who is interested: the Malay language has besan: relationship between parents whose children are married to each other.