There are several other materials you can use in the absence of milk. You can use lemon or lemon juice, baking soda mixed with water, vinegar or even grape juice. Basically, just anything acidic can be used as invisible ink. Also for the heating agent, you can make use of other heat sources such as sunlight or gas stove. However, you'll need the supervision of your parents if you are to make use of a gas stove as source of heat for this experiment.
Milk, lemon, vinegar, grape juice or any other acidic liquid, weakens the paper when applied. When you apply heat to it, the portion where you wrote your message oxidises and burns faster than the surrounding paper dry from the ink. The said portion will turn brown, thus revealing your secret message.