Short, specific and present tense. Everything else is optional.
Say that you know, say that you are not going anywhere, and offer one concrete thing. Avoid explaining what the situation means or what it will teach them — that is the part people remember badly.
Later is often better. The first week brings a flood of cards and messages; the fourth week brings almost none, and that is usually when it is hardest. A card that arrives a month in says something that a card arriving on day two cannot.
