As far as I know, they owe you the going rate for shadchanus al pi din.
I don’t know how you want to proceed, but the option of a din Torah is available to you. Ask a rov proficient in Choshen Mishpat.
It’s then your personal decision whether or not you wish to proceed. Obviously, you should first communicate openly with the parents.
I think speaking to the kids is, at best, a last resort. It could really lead to sholom bayis problems. (Although not paying the shadchan can too.)