Here are a few tips if you plan to visit the city with children:
1) Walk through the city center: city traffic is relatively light, and the center of Vientiane is fairly small. A short walk is the best way to catch the atmosphere of the city. Shop, visit a few temples, or stop for a drink in one of the many cafés.
2) Visit temples: the Vat Si Saket and the That Luang are must-see in Ventiane. It can be for the millions of miniature Buddhas in Vat Si Saket or for the golden color of That Luang that makes it look like treasure: children will like it.
3) Get high on the Patuxai: climb to the very top of the arch and enjoy the beautiful view on Vientiane.
4) Take a stroll through the park: the park on the banks of the Mekong is a great place to run and play. The park also features some fitness equipment, which my kids found very fun.
5) Enjoy western food in one of the many international restaurants: French, Spanish, Italian, you name it! If your children miss home food, seize the opportunity to give them a treat.
6) Plunge into the pool: the Ventiane swimming pool buildings and equipments are run-down but the pools and showers are clean. If your hotel doesn't have a pool, here's your chance to cool down among Lao people.
7) Visit the Buddha park: smaller and less impressive than its twin in Thailand (read my post about the Nong Khai Sculpture Park), but still great fun with kids!