Do you sometimes find it hard to know what to say to your child when they’re cross or upset, or having a tantrum?
If you use emotion words to validate does it sometimes make your child more cross?
Try this ultra simple but effective approach - literally repeat back to your child what they've just said. “You don’t want to leave the park” “You want that new dinosaur” “You don’t want to go to bed” “You don’t like it” etc...

No need to embellish it - less verbal input is better when your child is dysregulated, as they cannot process it and it may add to their overwhelm. Avoid the word "but..."! Just say it calmly and gently, and then physically be near to them in whatever way your child finds most soothing/least aggravating. You will know what they need.
You might worry about sounding patronising. There’s no need to go overboard with the soothing voice! Just do what is genuine to you and your child.
Hearing their own words packaged in a calm, compassionate voice is a powerful way to help them feel understood, and emotionally contained. It may help them to start to regulate their emotions in the moment... but don’t expect miracles! The main aim is to develop your child's emotional literacy, paving the way for them to learn to do it for themselves in the long-term.
Hearing their own words packaged in a calm, compassionate voice is a powerful way to help them feel understood, and emotionally contained.
Of course, this is just part of the puzzle - you may still need to calmly take actions to hold a boundary after/during this moment of connection, such as getting them out the door or up the stairs. If there are underlying anxieties about school (for example) you may need to think about a wider strategy at a calmer time.
Let me know how this goes for you, and if you have any troubleshooting questions.