For 22 years, I've been making sack lunches for my kids. Once a week or so, I will try to add some lunch ideas.
I fill small water bottles 1/3 full of water and freeze. If you have the freezer space, make up extras for the week. With lots of kids, this is more difficult. I typically put them in the night before-but I've been known to forget. Fill in the morning with cold water. I let the bottles sit out of the freezer for a few minutes before tightening the cap. When the bottle is cold, it is stiffer and hard to get a good seal. (Guess how I found that out.) You don't want your child's lunch water-logged.
If you have milk drinkers, I only had one, you can do the same. Freeze a much smaller amount, 1/4 or less the night before. Fill in the morning with more milk.
Of course juice boxes are plentiful, while convenient, they are expensive and unnecessary. My kids never really liked them anyway.