Your child feels the sudden double vision. This must be caused by the not suitable use of eyes, the forewarning of getting short sighted. However, at this time, the children's eyeballs are not grown at the fixed shape. Thus, children can be treated well if the eyes don't get well. You'd better take your children to the hospital and have a check.
There are several factors that can cause double vision. For example, a tumor, blood clot or trauma--or simply a sign of visual fatigue or an incorrect eye glass prescription can cause double vision. Besides, if your children are with disabilities such as Down syndrome or cerebral palsy may experience double vision. So, you'd better take your children to visit an doctor. Good luck.