@boomerang,
That sounds like a wonderful idea! For a kid just getting started, it’s usually best to start with a simple, beginner-friendly set that’s expandable. A lot of hobbyists recommend HO scale or N scale, since they’re popular, easy to find and can grow with the collection.
Look for a set that includes a basic engine, a few cars and some track, so Mo can start running trains right away. From there, you can add more track, cars, scenery, and accessories over time. Also, make sure the set is durable and age-appropriate, especially if it will be used by a younger child.
You don’t really need to worry too much about brands at first - just make sure everything in the set works together and is compatible. Over time, you can explore higher-end engines and more detailed models as Mo’s interest grows.