Call Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - http://support.apple.com/kb/HE57). Provide the serial number and specifications of the Mac. They may be able to provide a replacement set of discs for a fee. These discs will be for whatever OS was installed on the computer originally. I would not be optimistic about them having them for a computer that old and you may have to hunt on the used market. If you want an upgrade set you will definitely have to hunt on the used market.
There's several iBook models that can run Tiger or Leopard and it helps to be more specific as to your exact model and what system you want to run so we can be more specific.
Tiger or Leopard upgrade discs can be quite expensive ($100+) I also don't know what these kids are used to with computers. Both Tiger and Leopard are pretty out of date now, though some of us still use them happily. Still, if your kids are wanting to stream video online or surf sites smoothly you may find both hardware and software of an iBook are limiting compared to what they want to do and you may reconsider in light of what you would have to pay for the upgrades.
Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac". Write down what it says about "version"and report that here. Now continue to "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).