You want your child to be completely relaxed and not distracted. You need to bend the knee up and take control of their foot. You want to stretch down the Achilles tendon down the back, you don't want to be stretching the foot because then you're stretching the structures so it's good to take a good grip of the bone at the back of the ankle and feel that good stretch and use your arm to lever against the foot. You don't want to turn the foot out the way if anything you want it to come in a little bit. So a nice slow stretch, feeling the stretch all the way down calf and then taking above the knee you can hold that stretch and bring it all the way down for a nice long stretch, holding for 20 to 30 seconds.
If the child feels discomfort when you pull the stretch on, you can just take it off a bit or when you come down you can just let the knee relax. When you're stretching the calf you will feel better stretched the full knee extension. So again hold the ankle just above the bone at either side and give it a good pull down using your arm to lever on the foot and coming slowly down and hold.