Disclaimer: I am seventeen years old and so quite a bit older than this book's target audience. I often read books for younger kids, but my views will probably be different than those of your young ones. Take review with a grain of salt.
Second disclaimer: This book is the only one in the series that I have read. (It was a freebie from a summer reading program a few years back.) Perhaps reading the first two would have helped; perhaps not.
Third disclaimer: There are a few spoilers in here. I'll try not to give away too much though.
My overall impression wasn't the best. It certainly wasn't a bad book, but it wasn't one that I'd rant and rave over.
My first problem with the book was its version of morality. Evidently the kids had broken some rules in the previous book, and in this one they go ahead and steal the time machine from two of the kids' grandfather. There were no reprocussions to this. Instead, it was rationalized with an end-justifies-the-means mentality. After all, if stealing the invention could save Abraham Lincoln, isn't that a good thing? I don't think so. It's not that the kids are immoral but that they fail to take the moral stance.
The kids also lacked foresight and planning into their adventure into the past. They neglected to take along any food and decided to arrive in time in the middle of a busy street. Wise moves.
The issue of time travel is poorly dealt with. Perhaps it was better explained in earlier books in the series, but it sure wasn't here. Evidently their machine, TASC, needs a picture of the place they're going and reconstructs their molecules there. How does that work with the dates (which were entered seperately) you may ask? I have no idea. You would think that a picture from one date and a different input date would mess things up, but that wasn't approached. Maybe the picture they found just happened to have been taken on the right date. I'm not sure.
The kids never even think of the paradoxes of time travel, the simplest of which is the grandfather paradox. (What would happen if you killed your grandpa before he had your mom? You would never have been born to go back and kill him...Remember Back to the Future?) They conclude early on that they changed the past in a prior book, and then they go off to save Lincoln - doing it again. Not even one time does it occur to any of the kids that they could completely change their lives by saving Lincoln. Bright kids should come up with that.
I also wasn't impressed with the sensory detail. Sure, the author threw in some things here and there about clothing or what they ate (since they didn't bring their own food!), but overall I didn't get much of a sense of where the characters were. (However, that could also be because I skimmed too much since the book is so far below my level.) The storyline was the focus, not the setting.
Your kids may like it if they're into history. However, if they're too bright the time travel as presented may bug them, and they won't get too much more of an understanding of history. The actual information about the event was minimal and there are better books out there on the subject of Lincoln's assasination, even for young readers. (Try "In the Line of Fire," which is about all the assinations - successful and attempted if I remember right - of the presidents.) Just don't expect them to really get too much out of it - apart from reading time and a chance for you to get that much needed nap! (Oh, and read them in order.)
3 Stars out of 5