Thank you for your question. The first person I typically recommend is to ask their primary doctor of a referral for a[ marriage counselor. If not, Psychology today has a section for therapist in your area. What I suggest is to schedule an appointment, and ask many questions about the therapy process. A good therapist will explain informed consent, confidentiality, and one's style of therapy. After the first session, if your son and daughter-in-law feel good about the therapist, they should continue with the sessions. If they feel ambivalent, they can try another therapist.
Here is the website: http://therapists.psychologytoday.com/rms/
Another resource for a couple's therapist is through the California Association for marriage and family therapist.
Do you recommend they go alone at first or with the children?
They should go as a couple, first. After this meeting, the therapist may recommend bringing the children, if needed.