The problem with cat flaps is that cats other than your own can get in and out. They sometimes visit for a feed and maybe a sleep or just to scare the cr@p out of your little darling. Here is an article that might/might not help you.
My dad used to have a 'Garfield' toy backside hanging out of his car boot to look like a cat was stuck as was popular in the 80's - and he wondered why we chose to walk to school instead of catching a lift!
mushroom has the ideal deterrent against cats - toddlers. There are cats and toddlers who 'do' get on well but mostly as soon as a cat sees a toddler monster they will run and hide.