Black felt-tipped pens contain inks made from an enormous range of dyes, including yellows and deep purples. You need water-based pens (reject any that smell of other solvents). Write a note on soft, absorbent paper such as kitchen towel with the culprit's pen, and then mark this pen and several others with the names of the suspects. Cut the note into pieces for the children to investigate.
A few drops of water on the paper causes the ink to spread through the paper and the coloured dyes will separate. Doing the same with writing from the other pens will give different blots of colour. Match the patterns of colour.
Use a binocular or monocular microscope to match fibres by sight. Use a fibre from the crime, and a set of others. Colours, thickness, kinks and other peculiarities will help solve the case.
Press the culprit's shoe into a tray of soft modelling clay or damp sand.
Provide several trainers for pattern-matching. They can probably be matched on sight. (Lighting from the side throws the sole pattern into relief). A permanent record can be made by pouring a creamy plaster of Paris mixture into the impression. Leave it for 24 hours to set. Notice that plaster and water produce heat when they are mixed. That's from the chemical reaction.
Match the sole to shoes from the lost property box. Another piece of evidence!
Fingerprints take well on a very smooth surface, such as a glass tumbler.
Wash it thoroughly and then press the culprit's thumb onto the surface.
From now on, handle it like Inspector Morse, with a handkerchief. Provide some fingerprint records to match - use an ink pad and some willing colleagues. Label each with the name of the suspect, making sure you have included the culprit. Ask the children to use hand lenses to match the loops and whorls. They will need to shake a little talcum on to the tumbler print and blow it gently. It will stick to the pattern. Conviction looms!