if you want to lock your phone to one app when you hand it to them you can use app pinning in settings. otherwise look into creating a user profile and see what you can do to lock that down.

what type of phone/OS are your phone and computer?

there’s some hands-on controls too that may be useful but it starts with just giving them their own profile or login that’s ideally with as few permissions as possible. For android, there’s profiles which you may be able to lock down a bit more than your own profile. Apple had family mode settings that you may be able to turn on and off when they have the device. For a computer, same approach. Standard user account with whatever you installed using an admin account and lock down content via DNS like cloudflare for families, https://adguard-dns.io/public-dns.html, a self hosted one or something like https://nextdns.io/.