While there is no single word for the Yiddish mekhutn, מחותּן masc., and mekhetenste, מחותּנת'טע fem., the English phrase "my child's in-laws" covers the concept. Spanish does have a term though: consuegros, cf. compadres 'somebody's godfather and godmother'. I do love the mock question, vos bistu mir far a mekhutn?" (Do I know you?, literally, Who are you to me, are our kids married). The problem with coming up with an English term is that you'll have to explain it anyway, so why not use the Yiddish word? English has plenty of those.

Ceci n'est pas un seing.