S.,
Speaking as a former overweight child, I can tell you the last thing you want to do, regardless of your child's age, is tell them they are fat. I guarantee you they already know this, and having a parent tell them that will only be a cruel blow to their self esteem and fragile young egos, no matter how nicely you think you're going to put it. I had a father who was quite blunt, to put it nicely, and told me I was just too fat to be on the cheerleading squad in junior high. Let me tell you, that did the opposite of motivating me. That made me feel even worse about myself, which caused me to eat more, and you guessed it, gain more weight.
The best thing you can do, in my humble opinion, is just to start modeling the behavior you'd like to see in your child. That's what my mom did for me, and it worked. It was never brought up that our family was going on a diet, or even a lifestyle change. She just started serving much healthier foods and instituted family biking and hiking trips. The word's "we're doing this so we/you can lose weight" never crossed her lips, and that was the best thing she could have done.
I have to ask, since your posting was so small, what your motivation is for even wanting to tell your child that? If you think you're going to help them by telling you that, I promise you you're not.
Just be for your kids the kind of person you want them to be, and I promise you they will follow in your footsteps.
E.