Kittens trying to sneak outside

I have 2 kittens who are now almost 4 months old. For the past few weeks they have been sneaking outside anytime we go in or out. Luckily, we have seen them doing this and able to get them back in. But I'm afraid there will be a time when my husband or I will not see them and they will get stuck out there and possibly hit by a car.
What is the best way to train them that outside is off limits? I've been doing a little internet research and thought of putting a collar and bell around their neck so we can hear them run out, but read that this can cause psychological problems. The other thing I thought of is squirting them with a water bottle when they try to get out. The one problem with that is their litter box will eventually be in the basement and out of the second bedroom and don't want them to get too afraid that they won't go to the litter box in the basment (the stairs to the basement are by the door they are escaping from).
Any suggestions!!

All of the cats I've ever had have had collars with bells on them. It doesn't seem to bother them in the least bit, and it lets me know where they are. They actually seem to get upset when I take the collars off for some reason.

When I go in and out, it's become habit to make sure there are no cats near my feet. When I come in, if I suspect that one is there - the first thing they see poking through the opening is my foot. When they see that, they back off.

After enough repetition, they learn. If they think they will succeed, they will always try again. After enough failed attempts, most cats will quit trying. It just takes time, and you have to keep in on your mind every time you go in and out for awhile. The effort will pay off.

mine both have collars with bells on..... i like to hear them coming when they sneak up on me and it stops them catching so many birds. I just wondered why its considered to give them psycological problems???? I've not noticed any weird behaviour in my cats.

I read somewhere that the constant high pitched noise from the bells are too much on their sensitive ears. But I wanted to hear it from people who actually use the collars with bells, instead of some random website.
Thanks! I think I will try collars with bells, besides, they will look adorable and I can put them in different colors!

Collars with bells won't cause any psychological damage I'm sure. The only thing I would suggest though, is that you get the break-away collars for the safety of your kitties.

If collars with bells caused problems, mine would all be doomed! LOL. I put them, especially on the kittens, so that I can always find them.

The only problem with putting a collar on a cat (bell or not) is that a collar can catch on a tree branch or other object and can result in the cat becoming trapped and possibly strangled as a result.