Sometimes when you set a limit, your child may say, "Oh, OK," and quickly move on. Other times, they may act like you are ruining their life and dig their heals in.
If your child resists, try to see it as an opportunity for them to tolerate disappointment and potentially to process some other emotions. Let them cry or shriek, while you provide physical comfort, listen actively, and occasionally validate their feelings. After an outburst they will feel lighter (and more compliant).
If you don't have time to process your child's emotions fully, try a condensed process. If, for example you need to get to school, approach, set the limit, validate the feeling, and then set another limit:
"We need to leave now to get to school on time."
If you do this, likely these feelings will come up later. Try to acknowledge the feelings later, or even set another limit when there is more time to get the feelings out.