My kids both struggle with this. I try to reinforce how it feels when they smile naturally. We practice ahead of time for school pictures. I let them wear what they are comfortable in/ want to wear (within dress code) and we practice thinking of things that make them genuinely happy/laugh. So when they hear "smile" their brain triggers the happy memory to get a more natural smile. I also encourage them to practice in the mirror. Sometimes the muscle memory of what it feels like when they like what they look like helps.
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Mama to DS1 Punkin (2/04) and DS2 Boo (1/09)