Difficulty sitting still
Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
The core problem may be related to difficulty regulating impulsivity, and not understanding their own nature.
What Will Make It Worse
Reacting with frustration, and expecting them to start doing something that they haven't fully developed will do nothing to improve things.
Additionally, your angry energy towards them will escalate their emotions making it harder for them to be calm and still.
What The Child Needs
1. Self-regulation skills: The child needs to learn the usefulness of strategies for managing impulses, staying focused and channeling energy in appropriate ways. Teach them how deep breathing, body awareness, mindfulness and other self-regulating methods can make it easier for them to catch themselves moving around/being fidgety when they need to be still and focused, and take action to bring themselves back to stillness.
2. To identify their antsiness triggers: Help them brainstorm the situations or things that are making them squirmy and help them take the steps to remove these movement triggers. For example they may realize the ground is too hard and a little padding will help them settle into stillness, or that going right from a high dopamine game on a tablet to quiet work time is too difficult a transition for them to make right now.
3. Movement opportunities: Provide opportunities for movement and physical activity to help the child release excess energy before or after a period where they are expected to remain still.
How To Have The Conversation With Your Child
Find a time to sit with them in a comfortable place that is free of distractions.
Begin the conversation with empathy, expressing understanding for the child's need for movement. Let them know that it's normal to feel restless at times and that you are there to help them figure out how to handle the times at school, a restaurant, church, etc., that requires them to be still.
Then move on to working with the child on the "What The Child Needs" pointers above.
Over the following weeks, be sure to give them praise when they do the things you two talked about and are able to sit still.
The goal is build them up to a point where they can make the most out of the times to be active, silly, squirmy and wiggly, and then self-transition to being ready and able to be still.
"Sit still hard! Play hard!" ;)