The Washingon Post's excellent "Family Filmgoer" column always gives very specific details of what might be considered tough for some kids, so that parents can judge. Here is what the column said about "Lincoln":
"Three scenes make “Lincoln” problematic for some middle-schoolers. One shows soldiers fighting intensely but non-graphically with bayonets; another shows Lincoln riding through a battlefield seeing endless dead bodies; the third shows Robert watching as a wheelbarrow full of severed limbs is dumped near an army hospital. Characters smoke, drink and curse. The N-word is heard often, along with other racial insults. A marital fight between the Lincolns is upsetting."
Pretty specific and helpful, I thought. My 11-year-old wants to see it and I think it likely will be fine for her, but I will go see it myself first. If there is ever any doubt, I go see a film before she does -- that's truly the only way to know whether you want your child to see it, because you, not we or reviewers, really know your child best.