I can help with the secret code (mathematically known as cryptography) part. You could use some of the "codes" I describe as easy "Oh-you're-bored-here-try-this" games, or gathering games...
One of the easiest codes to do involves a two brooms with the exact same handle size (or other long and thin cylindrical objects), and a long strip of paper. The girls wrap the paper around the broom handle, and in a spiral shaped fashion so that the part of the handle is completed wrapped in paper. The girls can write their message, and remove the paper... and suddenly, the long strip of paper has transformed into a jumble of letters or your ... secret code. The tape can be passed to the other group (with matching broom handle size) who can wrap their broomstick with the paper, and the message will appear. :-) You could use items of varying diameters to show how it won't work unless the handles are the same size.
I think you can mix lemon juice and water, write on white paper with a toothpick, let the message dry, and "see" your secret message as you hold it up to the light...
Caesar was said to have used this code himself... This is just a "mapping" of letters, and most kids are familiar with it. In Caesar's case, A=C, B=D, C=E, D=F, etc. So the word BAD = DCF. You could have the girls team up, and have each team pick a letter to "map" to... A=J, A=Q, etc. and have them "encrypt" an short message. They could then give their encrypted message to another team to "decrypt" (encrypt and decrypt are the words used in the industry). The girls probably wouldn't pick A=C (if that is your example), (and definitely not A=A), so that leaves 24 letters left to try. Four girls could try six codes pretty easily... if the first word doesn't make sense, then they stop. You could turn it into a contest... Use parts of your promise and law for the messages, or work clues into the messages if you want to do them yourself. (Note: This is a very easy code to break. You might want to explain to the girls that very few people could read in Caesar's era, and so this code worked quite well...)
This is a more advanced version of the Caesar Cipher. Instead of picking A=C, B=D, you can map to random letters ... A=C and B=F and C=Z... Here is something you could give the girls...
We will use math and statistics to help us. Count how many times each letter of the alphabet appears in your message.
E is the most used letter in the English languge. Which letter is used most often in your code? Write an E underneath that letter every time it appears in your message.
The second most common letter is probably 'T'. Write an T underneath that letter every time it appears in your message.
A very common word in English is 'THE'. If this word appears several times in the message you should be able to guess what letter represents 'H'.
A strong clue in breaking a code is some knowledge of certain words that are likely to appear in it. For instance, this message mentions our camp. If you can find where the word 'GUIDE' appears, you will know what several more letters represent. You may also wish to look for the word 'CAMP'.
The most frequent letters used in the English language are: E, T, A, O, and N. See if you can use this information to find out which letters stand for O and N.
Solving the remaining letters is now up to you. What does the message say?
Technically, you don't even need to use letters in your code.. You could use little pictures or stickmen in various forms (Did anyone else read that Trixie Beldon mystery when they were little?) ... anything which can be recognzined and counted. :-)
The girls should be able to break your codes here... Make sure that these instructions will "work" on your message, though! :-)