We are still dealing with this, especially since my youngest went to first this year (last year K was half day, first is full day).
First of all, we have become much stricter about breakfast. Every day breakfast is either oatmeal, whole milk yogurt and granola, or eggs. They can have toast and fruit too, but they must eat the other items first. Also, we really encourage them to drink a big glass of water before they leave. Dehydration and hunger don't make for happy kiddos.
When I pick them up at school, I bring some water and a snack. Sometimes we hang out, but usually we go home, because my youngest is begging for us to leave. She likes school, but she likes home, too, and she is ready to leave school at the end of the day. So we go home, and typically they veg for a while...either they draw, or lay in their beds listening to music, or they read (or are read to). Then after a half hour or so, they kind of come out of the funk and want to play, talk, etc. After supper we go for a walk every night; it is nice exercise for all of us (and the dog won't sleep well unless she gets her walk!), and it is a physical release for them.
Snacks in school can also be an issue. DD#1s class only has stuff like pretzels or crackers, and I'm okay with that. DD#2's class has that, but also really sugary cereals, etc. So dd#2 brings her snack (her request) almost every day. Typically it is a piece of fruit, or a couple of peanut butter or cheese crackers. My kids *always* bring their lunch, and I try to make sure it has plenty of protein in it, to keep them going through the afternoon. In their classrooms, there are several kids who bring their lunch, and many do so because the families are vegetarian or vegan, or simply don't think the lunch is healthy. So it isn't a big deal for them.
Finally, like everyone else said, it is a huge developmental transition, and there is alot of stress (not necessarily bad stress, but stress nonetheless) on the kids, especially the younger set. Hang in there, it *does* get better!