You can also use a piece of gauze wrapped around the end of your finger.....I think it irritation is most likely the little bristles going everywhere in their tiny mouths that gets them upset.
Will have to be more vigilant about brushing
A good lesson for all cat owners - have your vet check for reaction to the pressure on the tooth/gums and swelling/bleeding gums - it indicates pain/problems. Please be sure to brush your cats' teeth & have them checked regularly. Kidney, heart, liver problems (among others) can be caused by tarter/bacterial build-up in their tiny mouths.