To address your second question first, yes, there are certainly some days that are not suitable for lounging by a pool here, heated or not. This month the mean temperature is in the low 50s, with an average max. temperature of 65°. In December that sinks to a mean of 46° with an average max. of 58° and January's at least a degree colder. It's not until April that you start getting average max. temperatures in the mid-high 70s again, which is when most hotel pools traditionally reopen.
As to why the major hotels don't have indoor pools, we'll take an educated guess at the fact that it's too expensive to maintain two sets of pools, plus swimming pools aren't small and would take up too much valuable real estate inside. The only Strip hotel (in fact, the only hotel, period) that has an indoor pool is the Tropicana, which actually has a novel heated indoor/outdoor pool that's open year-round.
But don't despair: Although many Las Vegas resorts close most of their pools during the chilly winter months, nearly all of them keep at least one pool open and heated, plus any adjacent hot tubs, so on a bright sunny day, your kids might still get to enjoy the outdoor-pool experience.
Another option for you to consider is, instead of traveling to Las Vegas during the depths of the winter, head to Reno. The best indoor pool in Nevada is found at John Ascuaga's Nugget in Sparks; the entire fifth floor of one of the two towers is dedicated to the big pool, Jacuzzis, and workout equipment. And with all the other amenities at Ascuaga's (not to mention downtown Sparks just outside the door), you barely have to go anywhere for your vacation. The Atlantis also has a good indoor pool, though it's not quite as big or attractive as the Nugget's.
What follows is a list of the Las Vegas resorts that maintain a year-round pool, together with the temperature that each one's heated to:
| Casino | Pool(s) open | Pool temperature |
| Bally's | No pool open, but hotel guests can use pool and Jacuzzi at Paris sister property, next door | 74°F |
| Bellagio | One of the five pools is open | 85°F |
| Caesars Palace | One pool is open, plus Jacuzzi | 80°F |
| Flamingo | Smaller pool is open | 72°F |
| Greek Isles | 24-hour heated pool | 85-95°F | Green Valley Ranch | Main pool open | 78°F |
| Hyatt Lake Las Vegas | Upper pool and kids' pool both open (plus hot tub) | 80°F |
| Las Vegas Hilton | One pool open | 82°F |
| Luxor | One pool open | 82°F |
| Mandalay Bay | South Lagoon pool (plus hot tubs) | 82°F |
| MGM Grand | One pool (plus two hot tubs) | 65-70°F |
| Monte Carlo | Main pool is currently open but will be closed for construction Dec. 3-Feb. 15 | 80°F |
| Paris | One pool (plus Jacuzzi) open - also open to guests of Bally's | 74°F |
| Red Rock | One pool open | 80-82°F |
| Rio | One pool open | 80°F | Ritz Carlton | One pool open | 83°F |
| Stratosphere | One pool open | 80°F |
| Tropicana | Indoor pool is open 24/7, year-round | 78-82°F |
| Venetian | All seven pools are open year-round | 80°F |
| Wynn | All five pools are open year-round | 82°F |
