Your child has strained the muscles and ligaments in their neck. A sudden, awkward movement can strain the neck. This often occurs with falls or car accidents or during certain sports. Everyday activities like using a computer or sleeping can also cause neck strain if they force the neck to be in an awkward position for a long time.
It is common for neck pain to get worse for a day or two after an injury, but it should start to feel better after that. Your child may have more pain and stiffness for several days before it gets better. This is expected. It may take a few weeks or longer for it to heal completely. Good home treatment can help your child get better faster and avoid future neck problems.