My four-month-old kittens both managed to get out through the cat flap today. I didn't think they would be able to open it yet because it's actually a small dog flap that the previous owners of the house left and it's quite heavy. I thought we had another month at least before they would be able to get through it. I've locked it for now but I would prefer to leave it unlocked as my adult cat uses it. Will the kittens be safe going out so young? With my other cats I've kept them in until six months old.