If you have a child studying and living away from home, they may automatically be covered against theft or loss as part of your home insurance policy under its 'temporarily removed from the home' section.
Many policies have this cover (but do check yours), as long as your home is your child's main permanent address (in other words, they'll return to your home when not a student). However, it only applies while the contents are in their accommodation (or indeed at your home).
If you want cover for mobiles or laptops, or other items your child normally wears or carries away from your home, or their accommodation, you can add an 'all risks' or 'unspecified personal possessions' section to your policy. This covers your child's belongings while they're out and about, but it typically comes at an added cost.
Alternatively, if you prefer your child to have their own policy, see our Contents insurance for tenants guide.