It's a very simple process. All you would need is to buy a NDS Flashcart. Some are better than others. For the DS Lite/DS I would reccomend purchasing a CycloDS or an Acekard 2.
You would also need to purchase a Micro SD Card to place the cart firmware and roms on.
Flashcarts can be purchased on many mod sites, and there are DSi compatible flashcarts if you choose to get one of those for your daughter instead.