Children interested in model trains will have different wants and needs than adults who pursue this hobby. Care must be taken to give age-appropriate trains to children. Children under eight years of age should not be given electric-rail systems; they should be happy with trains that are pulled along the track by hand or battery operated trains. Brio trains have been around at lest since I was a kid. I loved them. Lego offers their Duplo trains that play well with their construction blocks.
Thomas Trains
Small children love Thomas the Tank Engine and his friends. There are many "Thomas" children's toys. Duplo has a Thomas set in its product line. As children mature, they may want "real" electric trains. Fortunately Thomas and a number of his friends are available in O, HO, and N Scale. Model train enthusiasts who have small children may want to acquire these trains in the scale they model and put them on the layout as a means of stimulating a child's interest in their hobby.
Trains for Older Children
As children get older their interests change. As children outgrow Thomas, they may find an interest in the Polar Express or the Hogwarts Express from the Harry Potter series of novels and films. Lionel manufacturers both Hogwarts Express and Polar Express sets. These are O scale and do require some space to set up.
Trains and Education
The growth of air travel has reduced the cultural significance of railroad trains in the United States over the past 50 years. To a lesser extent this has happened in other parts of the world also. The emergence of Thomas the Tank Engine in popular culture over the past two decades has renewed interest among young people in trains and railroads. I personally hope to see this trend continue. Children have always loved dinosaurs. Why can't they love trains too. Trains are part of our cultural history. There is much children can learn if they develop a serious interest in model railroading.