Acceptance can be a complex practice to teach a child. You might already be practicing this without realizing it is a practice of acceptance.
One opportunity to practice acceptance is when the child does not get what they want (e.g hot chocolate). They might be crying, mad or some other emotion. However they are feeling, allow them to have time with that emotion. Then inquire,
Can you share with me how you are feeling right now? I know you were excited to get XYZ, let's take some deep breaths together (count three aloud).
How do you feel now?
Our breath can be a grounding tool to help center our mind and body. You might share with the child, "I know there was excitement for ....and you are feeling ...., we can choose to feel this way this way the rest of the day or try to be okay in not getting..... so that we can enjoy other parts of our day. Can we try that?"
Should your child continue to be upset, can you accept how they are feeling?