My favorite handgun box has a push-button Simplex lock and is reasonably secure. It is made by American Security (AmSec), a manufacturer of safes. A locking file cabinet might be a good secure hiding place if it’s made by a real office furniture company like Steelcase… especially if the kids think it only holds filed papers like taxes, paid bills, insurance.
Give her a copy of Massad Ayoob‘s 1986 booklet “Gun-Proof Your Children!” which is reprinted as a bonus section in many editions of his “Handgun Primer” book. He has another book titled “Gun Safety in the Home” that likely includes the same info about educating kids, satisfying their curiosity, reducing the temptation of forbidden fruit. Look on abebooks.com or Alibris or Amazon. I think he’s posted a free video about gun-proofing children.
NRA also publishes info about securing guns in the home, educating kids, dealing with the kids’ friends and acquaintances.
A kid-sized .22 rifle might be a good second weapon for her to buy. I’d lean towards single shot, bolt-action, or lever gun - avoiding the temptation to spray and pray.
Best physical security would be on-person concealed carry during waking hours, locking it away only when sleeping or going to prohibited locations. Do not give kids and their friends opportunities to spend unsupervised hours trying to access the stored weapon in its box or safe.
Some of the popular quick-access vaults / safes / boxes have been easily defeated by kids using simple tools (magnet, hammer, screwdriver, pliers, or just banging the corner with a shoe!). Some years ago a mom challenged her 12 year old then posted scathing reviews of several of them describing how easily he broke them open. Lock-picking instructions abound on the internet… so I am not a fan of most “keyed backup access” for electronic security devices.