The basic idea of some sort of pigment dissolved in a solvent remains a common factor among all inks. Distribution of the tea bags and iron tablets is easy, but you should have another adult volunteer dispense the hot water to the students instead of letting them pour it. An electric tea kettle or coffee pot can be used to heat the water. I purchased Ferrous Sulfate Capsules T.D. by Geneva. These have 50 mg of iron and contain 100 capsules.
Students can open the capsules easily and dump the granules into the cup.
If only tablets are available, you should crush them before adding to the cup. Put them in a baggie and tap with a hammer or some other heavy object. Cutting the straw is another critical step. The point of the pen must be relatively sharp to work well. Allow students to experiment with point shape.