What would be the most common long term effects of sustaining a pretty bad head injury for a child?

Depends on degree: The degree of injury can give you an indication of the potential long term effects. Those patients requiring surgery with prolonged coma before "waking up" have more extensive neurological dysfunction. Those that wake up can have: cognitive deficits, developmental delay, focal weakness, spasticity or increased tone. Hydrocephalus can develop years later as a result of cerebral degeneration.