How do my husband and I give up our parental rights and force our child's guardians to adopt?

You need advice of an attorney with family law and/or adoption law experience; this is a complex situation to begin with, and something does not seem "right" from what your write. For example, no one can "legally kidnap" someone. If parents are unfit, it is possible that the legal system may give a child's care to someone else (e.g. a foster parent; a relative), but that's with essentially a court determination; there is no way that his parents could have taken the child w/out a court order and without your consent.

Also, there is no way to force anyone to adopt a child. It may be possible to terminate your parental rights, though that is doubtful without either someone else who chooses to adopt or, again, some court determination of unfitness.

You need expert legal help to sort this situation out. If you can't afford an attorney, try Legal Services; they may be able to help you or refer you to an agency which can. But don't try to resolve this on your own.