Not sure how long you are flying...but I have flown domestic and international w/my now 3 1/2 about 8 times and have never had an issue with bringing any of the following:
Fruit juice in a sippy cup (it was over 4 oz and they just looked at it, never opened it - they were able to determine it was juice by looking through the thick plastic - don't even get me going!!!!!)
Cookies
Fruit Bars (like Nutragrain Bars)
Dry Cereal
Applesauce in little cup ( you know those single servings - don't forget the plastic spoon!!)
Candy
Pretzels in a zip lock
Goldfish
Never brought anything that needed to be kept cold. That's a bit to much to lug around. Especially since you're gonna haul things to keep em busy!
The only time I was slapped on the wrist was when I tried to sneak a bottle of water through security. If you have to have the liquids - just save the hassle and buy it at the airport - save yourself the headache !!!