There are a number of products on the market to unclog drains, but the thing to remember is you need to buy the one that is specifically for the toilet and the type of system you have. If you use a product that isn't for your system you risk more clogs and very expensive damage. You should start by plunging the toilet to see if it is something simple. Once this is done you want to make sure to cover the floors with newspaper to eliminate slick surface area from the splashing toilet. This will help with cleanup later. If you suspect something was dropped into the drain from a child you might try a wire hanger or snake to unplug the clog. My son sank a ship in our toilet once and a wire hanger solved the problem right up. If you want to try chemicals you could start with a baking soda and vinegar method which is a great natural solution. You would need a box of baking soda 8 ounces and a 32 ounce bottle of vinegar. Pour the baking soda into the toilet. Slowly pour the vinegar over the soda and stop when it starts to foam a lot to avoid it running over. Once you have used all the vinegar let it sit 15 minutes or so. You will then add a gallon of hot tap water to the bowl and let it sit overnight. You may need to do this more than once.