This was a good short stop for our family with 2 little boys (4,6). They enjoyed seeing the trains, but after a short time, it became a bit boring to them. Most of them weren't moving so there wasn't much excitement. There was a Thomas train track room which captured their attention, but since we can visit that anywhere (like our own room) I didn't want them to stay there long.
My husband and I were fascinated with the details on the tracks and the little train cars.
When my son was a toddler and preschooler, this was one of his favorite stops while in San Diego. It's a small museum in Balboa Park, but the train models are pretty elaborate. There is one little section with a train table that kids can get their little hands on. There are plenty of volunteers keeping the model trains running along and working on the layouts. They will often engage the kids and answer questions as well. Watch out for the gift shop in the entrance.
The Railroad museum is great for kids although my son liked it more than my 7 year old daughter. There are several running trains and the kids can see the trains moving. There is also a Lego train that is huge and very entertaining, because of the detail that Legoland put into it. The museum is in Balboa park which is a great place for kids to run around outside.
Four scale-model railroad layouts detail the geography and development of the railroad industry in Southern California. Your kids will love the hands-on model railroad.