Why does the message have to be in English?
Messages that are meant to be understood across languages are usually encoded visually. Think of the pictograms used to direct people on airports, or the comic-book-like saftey instructions in the nets at the backs of airplane seats. Or think of making drinking gestures.
If I wanted to communicate a message to those kids, I would draw a comic, in very simple images, using objects from their context. They might puzzle over this for a while, not having seen drawings before, but I would guess they will make sense of it very much faster than a written text.
There are many comic books and graphic novels without text. Some even illustrate abstract concepts such as evolution (Jens Harder, Alpha ... directions). Of course, the meaning of a wordless comic will become more vague and ambivalent, the more complex the message is. But words aren't exact in their meaning either, and abstract concepts such as freedom or love are interpreted quite differently by different people and cultures. The more complex the message is, the more difficult it will be to make it understood by those children, no matter if it is encoded in words or images or some other means.
If the message must be written, then think about how people learn languages most easily. Current language teaching uses images or movies in connection with written or spoken text, the images showing what the words mean. One of the best examples is the Rosetta Stone program, which begins with children ("boy", "girl") playing ("the children jump") and other simple objects (colors, fruits etc.). The program is completely in the language you are learning, with no grammar or other explanations, just presenting language and images in the same way that parents teach language to children. The astronauts use this program to learn Russian in a few months.
Everything else is highly unlikely to work in the real world. People having to deal with survival won't form a community of scholars that will attempt to make sense of a sheet of paper covered in what to them will look like some random texture. How are they even supposed to know that writing is a form of encoding messages? (Also, how many linguists are there in a given population? Maybe there are one or two kids with that kind of talent and interest in your group, but two kids did not crack the hieroglyphs.)
Also, a complex language might not develop in a few years. It is very well possible that those kids will be communicating in grunts and gestures and have no concept of language as we know it at all; or that their "culture" will be so different from ours that an accurate translation of a given English sentence will be impossible without making those kids familiar with out culture and its concepts first.
It seems to me that if the person abandoning those kids on that island plans to later give them a message, he would be stupid not to give them the means to decode it also. That is, to increase the probability of success of his plan he would give those kids a means to learn English (or whatever language he wants to use).
