How to reduce their body temperatures?

Cool things. Some medicines like ibuprofen and paracetamol can reduce fever when people are unwell. Cool things to drink, fanning and sponging cool wash cloths on the body (especially on the neck and in the armpits) are good non medicine things to try.
Better to let it be! If they have a fever it is ideal to NOT lower it! Fever is how out bodies fight infections; our immune system works better and microbes don't survive as well. Lowering fever sabotages our immune systems and prolongs infection. If absolutely miserable can take some paracetamol. If you have access to homeopathic medicines you can try Belladonna, which can lower fever without suppressing immunity.