1. Don't be too harsh, they are children after all. Make sure your lesson plan fits the grade level of the class, but also be prepared to adjust it along the way. If the whole class has a tough time with it, it's not their fault.
2. Do be honest, but be gentle. Children have to grow from their mistakes. One criticism I find myself saying a lot is:"We need to stop with the scribbling, we're getting too old for this. Why don't you watch how I color? Make sure you fill in all the white areas."
3. Do allow the students the opportunity to work on their project more in order to make improvements based on your critique.
4. Don't allow students to be hard on themselves. Tears are definitely not necessary.
5. Do make critiques fun. I have a plastic blow up cube that I wrote on each side one of the Elements of Art. We toss the cube around the room like MUM and whatever side is facing up; the students discuss how they used that element in their art.
6. Do self-critiquing and peer critiques. You can have a worksheet you pass out at the end of each project and ask the students what they felt was the most fun part of the project, what they struggled on, what they learned. You can also pair up students who based on their level of achievment can truly help eachother.