Getting cats to use a cat door inside the house

We have two cats, both almost 2 years old. We recently moved their litter box to our basement. We've kept the basement door open for them and they go down there with no problem. We can't leave the door open as a permanent solution because we have a 7 month old who will be crawling soon. As a remedy, my husband installed a cat door in the basement door. We taped the flap up to get them used to going in and out of the it and they seemed to do it with no problem. So we then tried closing the cat door so they can get used to going in and out of it. Here lies the problem. They refuse to go in and out of it. It's like they don't know what to do. I've tried pushing them through it but they sort of look at me like I'm nuts.

Any suggestions to get them to use the door? Should I try putting their food in the basement too?

Re: Getting cats to use a cat door inside the house

try using treats