Talk About It
You can explain that the photo session is like a fun game where the person with the camera sets the rules. Looking through the family pictures also helps to understand the process and why it is important for the whole family.
Choose a proper outfit
And try to negotiate. Usually, kids like having a choice, so let them choose from two options you like. I know it may take time; you may need to start earlier. Do not worry if you fail; we do not want on the photos a disappointed kid wearing a nice outfit.
Here you can find more practical advice about photo session outfits.
Think of a proper schedule
Ensure your child is well-rested and has a good schedule for the photo session. This will help everyone feel relaxed and get a beautiful result. I always have snacks and water for my photo sessions, but make sure you have something special for your kiddo if it is needed.
Let the kids have fun
Stay calm and patient during the photo session, even if everything goes contrary to the initial plan.
So have fun, too!