When you look up binocular specs you may see something like “IPD 54~72mm”. So what does this mean ?. IPD stands for Inter Pupillary Distance. This is the distance in mm from the centre of one eye piece to the other. In the example above, it means when the binoculars are closed the IPD is 54mm, and when they are fully open the IPD is 72mm.
If chosing binoculars for children, an IPD of less than 55mm is ideal