My cat is dumb - so what's new?

We keep the cat food and the shitty litter tray in the garage so that
certain young crawling babies don't eat cat biscuits or cat faeces
(ewww). Our garage has internal access to the house and we like to keep
the door closed to stop the baby from getting in and the cat turd odor
from getting out. So dh installed a cat flap door so that we don't have
to open and close it every 30 seconds ('cause for some reason cats
*need* to go in and out all.day.long). The only problem is that the
cats refuse to use it the way that it is intended, ie with the flap
down. They are too dumb or scared to realise that they can push their
way through and so the flap is taped open to leave a hole. Tonight the
tape broke and the flap fell down. So Crazy Cat spent 15 minutes
headbutting it before I realised that she wasn't just being a pain in
the arse LOL. I've taped it back up but don't know how long it will
hold - I wish they would learn how to use the thing!