Depends on what they like and what time of year they plan on going. Negril probably has the best beaches but is also the most touristy. The public beach is lined with vendors, bars, restaurants etc. The "resort" beaches tend to be manicured very nicely. I have been to Jamaica many times and have brought my children (ages 4-13) along for the experiences. Our last trip was a little less than a year ago.
Couples and Beaches resorts are all well kept and all inclusive. They do everything they can to keep you on the resort with the exception of some "guided tours" they offer to different tourist destinations. The 1st time I went to Jamaica I pretty much stayed on the resort and had a perfectly good time. I got to meet a lot of freindly people from Germany, UK, Canada and such but the only Jamaicans I met were the ones working. I wanted to know more about Jamaica so I kept in touch with a few of the "workers" I met and planned another trip.
I now have contacts in Jamaica that have taken me to some of the most beautiful parts of the island (places that the tourists don't go). My eldest son and I have gone fishing with a local fisherman, we have visited a meditation center in Cockpit country. We have been taken into local villages where we gave away toys to children who were forever grateful. We stopped at local restaurants (some seat 4 or 5 people) and enjoyed dishes like curried goat, ackee and saltfish, rice and peas, dumplings, jerk chicken and pork. We were able to float down the Martha Brae river on a 30 foot bamboo raft. Experienced the glowing waters of Glistening Waters. Ventured into the caves and swam in the "healing" waters of Roaring River.
We did all of the above without getting mugged or hassled in any way. Would I walk down a dark alley in Kingston. No, but I wouldn't do that in Jersey City either. Will you be offered marajuana to purchase? If you stay on the resort, probably not but if you go off, there is a good chance. All you have to do is decline and they will leave you alone. Vendors can be very aggressive at times, grabbing you by the arm and bringing you into their store/hut/tent etc. Higgling or bargaining is the norm at these shops. Don't be afraid to make counter-offers for souvineers. There is so much more I can tell you about Jamaica and If you or your future SIL want more information, give me a PM. I could supply you with a contact or 2 in Jamaica that will help them have a wonderful time if they want to go off property.
Adam aka Big Dude