The beauty of going into the promotional items business is that there are virtually thousands of items that can cater to a multitude of customers and clients. Even if you should just pick one particular market, say, the children's market, you will still have a diverse target range, especially if you consider the income bracket (of the parents, of course!), ages, and regional locations of these children. You will also have the added bonus of being able to market some side wares to these children's parents. And best of all, it can work vice-versa - you can produce products to the adults and have the added benefit of having children-focused products available to the discerning parents as well.
Promotional items are so unique as you never know where they will end up, or to how much further than the recipient they will stretch. Pens are always useful, apart from that they are leaving places or people accidentally pick them up. A pen that is given away might be taken home, and then somebody takes it to their working environment. Perhaps another member of staff picks it up and then they take it into their home, all this time the pen is acting as a subtle form of advertising.