Always begin by dating each entry. It will nag you later if you don't know the date.
Don't censor yourself. The "critical editor" is a silencing voice that will make you put down the diary. Children want to know. They're not going to red-pen entries later and say, "Mom misspelled this word." Trying not to reread for a while can be very helpful.
Go for passion rather than polish. A diary doesn't have to include perfectly crafted sentences or paragraphs. It's about connecting with your children. It's OK to ramble. Editing isn't necessary.
Quote liberally. Children enjoy knowing exactly what they said. The poetic things they say make them feel they have a special voice.
Don't give your children advice in your diaries. They don't want to read a lecture. But if they give you advice, write it down.
If possible, buy quality blank books, with hard covers and good paper. Spiral bindings, which lie flat, make writing easier. Portability is also a plus. You can tuck it in a diaper bag or a backpack and take advantage of a few moments of downtime before a soccer game starts.
Store diaries in fireproof containers.