I think you will enjoy your stay at Melia Tropical and for traveling with your inlaws and 4 year old should be a wonderful time for them to enjoy their 4 year old grandchild. The Melia caters to small children and have their own Flintstones section.
For comfortable accommodations, I would also check out Punta Cana Resort & Club that sometimes has great rates and is not all inclusive. May I recommend you do the Punta Cana ecological foundation walk and visit several of the springs, and combine it with a horseriding sojourn (do this one first).
DR1 - Hotels - Punta Cana Resort
I would recommend then that you rent a car (recommend
www.dr1.com/national) and then venture out to Bayahibe. Book an excursion to Saona Island from one of the operations in town. Combine the snorkeling excursion with a drive into Altos de Chavon in La Romana. Ask there for authorization to travel into La Marina de Casa de Campo. Spend 2 nights in Bayahibe. Travel also from there to Dominicus beach. Go to the public beach end beside the Dominicus Palace. Two hour drive to get here, so get started real early to be back before nighttime.
Visit the Cueva de las Maravillas on the way to Santo Domingo. Half an hour from La Romana city. Shop for art at Tamburini or Eduardo's in La Romana. Have a snack at Trigo de Oro bakery in la Romana.
DR is a great country to tour staying at small hotels, but my experience is that this is best done as a couple, and not with a 4 year old in tow. See
Dominican Republic directory of businesses, products and services end check out the small hotels options around the country.