My DD just turned three and I have always packed her lunch. What I've done is to pack hot lunches (I heat them up in a cooker on the stove just before we leave) and on cold days put little handwarmers in her insulated lunch bag. The lunch bag we have has 2 compartments so I'll keep one compartment just for warm things. She's never complained about having cold food and she finds things hotter than us anyway, so it seems to have worked.
But we eat a lot of rice and beans, so she gets rice and beans (black, garbanzo, lentils), steamed veggies, mac-n-cheese, pasta and sauce (we use protein-rich pasta, so even though it's meatless she gets a chunk of protein), bean chili, fruit (pear, berries, papaya), yogurt is always a good choice, other types of sandwiches (use fiber and protein-rich bread and think of other spreads, healthy sandwich meat, jam, cheese), healthy chips (kowalskis and trader joes both sell lentil chips, which contain fiber and protein, as opposed to potato or corn which only have carbs), clif bars for kids (less sugar, more nutrients). The nice thing about allowing yourself to send "warm" food is that it allows for more variety.
The one thing I won't do is send her with meat items, but that's more because I'm super paranoid about meat going bad! Good luck!