A privacy bolt will do the trick so long as it can be installed in a way that normal attempts to open the door won't break the door apart, no matter how unreasonable "normal" is in your home.
Hollow doors have solid frames within. In the above photo, the bolt has two brackets. The one nearer the edge of the door should be screwed into the solid frame, using appropriate screws of maximum possible length. This requires some experience and finesse, knowing what the frame is made of and whether or how to drill it. The other bracket should be installed with hollow door anchors. Double doors will need two of these, one of which could be kept latched at all times.
Another way if the doors open inwards is to put a couple of U-shaped brackets on either side of the door frame and hang a broom stick between them.
Another approach that is more robust is to sandwich the door with metal plates about 3 inches by 8 inches in the area of the bolts or locks. The plates on both sides of the door are bolted to one another through the door, and any locking hardware is attached to the plates. This spreads the forces of any attempt to open the door across a wider area. Hopefully this isn't necessary, IE, the doors aren't that sh*tty and your kids aren't that determined?