The Oregon Zoo's Zoolights is a really fun place to take your kids. The zoo is open at night and some of the animals that are normally lethargic during the day are a little more playful...especially the tigers. You can take the train around the zoo loop and they even bring out the steam engine for zoolights. All of the trains are even decorated with Christmas lights. There are Christmas lights everywhere at the Oregon Zoo for Zoolights! There are two things to keep in mind if you visit the Zoolights: weather and crowds.
The number of people that go through Zoolights is amazing. The parking lot fills up and the Zoo offers a shuttle from the Sylvan area over to the zoo. Trimet is also a great option for getting to the Zoo and avoiding the nightmare of the parking hassle (it will also save the $2 per vehicle parking fee). The best way to avoid some of these crowds is to show up at 4:15 or 4:30. The Zoolights officially opens at 5 pm but if you have small children it will take you a while to get out of the car, bundle everybody up, and buy your tickets. Plus, once you do all of that the immediate need for a bathroom break seems inevitable.
If you decide to ride the train you will definitely want to get there early. While you are waiting in line the sun will finish its descent and the train ride will still be completely enjoyable and in the dark. Don't forget that while you are waiting in line for the train you are pretty much standing still which means the cold feels even colder. Hats, coats and gloves are an absolute must to keep warm. Scarves and blankets would not be a bad idea, either.
Also, the Zoo sells hot chocolate and coffee but they do not provide lids or straws for their drinks as the lids and straws pose a health risk to the animals. If you think you might invest in some hot chocolate for your young children (or if you have a drinking problem like I do) you might want to bring your own travel coffee mug to transfer the drink into and avoid some spills.
Our family has been lucky enough to have a Zoo membership so the Zoolights has been free for us. With this advantage we have even done the Zoolights in two different nights because it was so cold after waiting for the train (riding the steamy was a HUGE priority for us) that our kids were miserable so we left. There was a different evening that was a little bit warmer so we decided to brave the crowds a second time and had a wonderful time walking around, looking at the animals and enjoying the lights.