What are the risks if a child who has been taking strattera (atomoxetine) for a 2 years stops taking it suddenly?

Not Severe. There could be some dirsuption in the mood along with complaints of not feeling well. There is no know severe risk. However, it is always best to stop medicines gradually so that there is as little disruption as possible. Your physician needs to be involved. Plus, stopping the medicine will mean that any benefit from the medicine will be lost over a period of days.
Withdrawal symptoms. Following symptoms may occur. Irritability, nausea, vomiting, anxiety, diarrhea, insomnia, agitation, tremors, flu like symptoms, loss of appetite, headache, abdominal pain, shock like "brain zaps".