Passcode not immediately required

If you have Face ID turned on and you swipe upwards on the screen, it is not going to ask for the passcode as your face has unlocked the device. Lock your iPhone by pressing the side button and waiting for the screen to go dark. Then take your thumb and cover the notch in the top center of the phone to cover the Face ID camera and swipe the screen up with your other hand. It should remain locked and prompt you for your passcode. It should not open. The phone will only prompt you for your passcode if Face ID is not working, your are not looking into the camera, or you have powered the device off and back on again. It will prompt for the passcode the first time after powering it back on. Otherwise, Face ID will unlock the phone. It would sound by your initial description that it is working as it should, unless you are not looking at the phone when you swipe upwards.

There are 2 settings:

• Settings/Face ID & Passcode: Require Passcode
• Settings/Display & Brightness - Auto Lock

The 2nd one determines how soon the screen locks when left idle. Not also that if you lock the phone but are still looking at the screen it will unlock.

If you don’t want her to change the screen time passcode don’t give her the Apple ID password for the phone, or include her phone in Family Sharing, then see this article on parental controls→Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Disable Face ID and see how the "require passcode immediately" behaves.

FaceID enters the passcode instead of you entering it manually. However,

Your device will require your passcode when you do the following:

• Turn on or restart your device
• Press the Home button or swipe up to unlock your device (you can change this)
• Update your software
• Erase your device
• View or change passcode settings
• Install iOS or iPadOS Configuration profiles

Are you sure that your kid(s) don't simply know your passcode?

I would suggest changing your iPhone passcode.

and if you think your passcode is/was known, then also reset Face ID and set it up again.

If you and your kid look enough alike, entering the passcode could train Face ID to recognize her face as well.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to latest iOS/iPadOS, unless you have intentionally downloaded spurious softwares or unauthorized apps directly from the internet and installed on your device or/and have Jail Broken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jail Break
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing

If you have enabled Face ID, your kids can't open it.