1. The Title: This is a bit obvious, but if you're keeping records in a journal or on a blog, it's considerably more necessary than if you're leaving your notes on the inside cover of the book itself. Recording the title can be useful in other ways, particularly if the book comes in multiple editions, or if you're reading a translation of some kind. Something is communicated to me, for example, when a reader claims to have read Demons rather than The Possessed.
2. The Date: To know that my mother read a particular book while she was pregnant with me, or when we were on a summer vacation, or while she was between jobs, is interesting to me personally. Books then provide a kind of literary timeline to a life. For the rest of the world, this may have no significance at all, but your children will consider nothing more fascinating in the years to come than the realization that you were thinking, feeling, and experiencing all manner of things they were unaware of while they were right under your nose. There are other perfectly decent reasons for recording dates, but I find this one is my favorite.
3. Where It Came From: Many of the books lining my shelves were gifts. Some were unintentional gifts (i.e. they were loaned to me and never returned). There are a few which I remember purchasing with perfect vividness. You Shall Know Our Velocity!, by Dave Eggers, with Lisa and Tara during a spontaneous trip to Fullerton back in the day when that city felt exotic to my desert experience; or The Princess and the Goblin, which I hunted for in an attempt to begin a collection of first editions.
4. Favorite Passages: Whether by page number (accounting for the edition) or transcription, pointing our your favorite passages is a way of developing a roadmap for future readers. If these are passages you have considered long after you first picked up the book, it may even be something recognizable to your children - if not word for word, then in the manner by which their content has influenced you in some way.
5. What You Thought: Why is this book important to you in particular? Why should your children read it - or avoid it? Recording your own thoughts is not just a way for your children to converse with your own reading experiences across time, but it's also a way for you to synthesize in your own mind what might otherwise be a passing impression. "What you thought" is a broad prescription for any number of written observations. Take time with this, and enjoy it.