Make shell script contents difficult to read
Option 1: use the shell script compiler to turn the script into a binary executable. The binary will still contain the script (in highly obscured form), but unlike a regular script, you can set the file permissions so the account the kids are using doesn't have read access to it (just execute).

Option 2: Encrypt the safe code using the "passwords", store the encrypted code in the script, and use what the kids enter to decrypt it. Here's an encryption process you could use with three passwords, "sekrit1", "hunter2", and "p4ssw0rd3":

$ echo '12 left, 25 right, 9 left' | openssl enc -aes256 -base64 -pass "pass:sekrit1|hunter2|p4ssw0rd3"
U2FsdGVkX18IFQAaSjEv2AJJ16z6wjROjcHiqHWGvji3MEsmcHwPgu3MQeh2O+c1


Then in the script:

read -p "Enter the first password: " pass1
read -p "Enter the second password: " pass2
read -p "Enter the third password: " pass3
encrypted="U2FsdGVkX18IFQAaSjEv2AJJ16z6wjROjcHiqHWGvji3MEsmcHwPgu3MQeh2O+c1"
if result=$(echo "$encrypted" | openssl enc -d -aes256 -base64 -pass "pass:$pass1|$pass2|$pass3" 2&gt;/dev/null); then
    echo "The safe combination is $result"
else
    echo "At least one password is wrong!"
fi


If you want to give the kids password-by-password feedback, you could add checking the hashes of the passwords as they're entered:

$ echo "sekrit1" | shasum
b19fb68c28bff07cf8fcc7c53ab48c5d6f41e993  -


In script:

read -p "Enter the first password: " pass1
if [ "$(echo "$pass1" | shasum)" = "b19fb68c28bff07cf8fcc7c53ab48c5d6f41e993  -" ]; then
        echo "Correct so far..."
else
        echo "Wrong!"
        exit
fi
...etc


BTW, this isn't a really secure way to store passwords; proper secure password hashes are designed to be slow and use "salt". But this should be secure enough for some kids.