For city lights and skyline views around sunset, start by comparing MaHanakhon Sky Walk at King Power. From its panoramic deck over the city, going up when malls across downtown switch on their December lights lets you watch the view shift from daylight to a lit-up skyline.
To fill two or three hours with children between mall plans, look at SEA LIFE Bangkok & Madame Tussauds Bangkok. As an indoor attraction inside Siam Paragon, it fits naturally into a Christmas-season shopping route.
For a full day with children, compare Safari World Bangkok; for a relaxed tour of an extensive outdoor site, compare Muang Boran (Ancient City). Dry-season weather keeps both comfortable to move through outdoors, making either a good pick for a full day of a three-night trip.
To close out a day in the same direction as Muang Boran, also look at The Erawan Museum. Located toward Samut Prakan, it pairs easily with Muang Boran in one day, and touring its distinctive interior is manageable for a family.
To fill an evening with a performance, compare Calypso Cabaret Show. It performs inside Asiatique, which puts up trees and light displays every December, so a stroll past the decorations before or after the show fits naturally.
To add energy to a sightseeing-heavy schedule, look at Flow House Bangkok; to fill a gap between mall plans, compare Jurassic World The Experience Bangkok. Before booking, check entry time slots, session format, and book early given peak-season crowds.