Technically there is no term like severe myopia
, it is either simple myopia or high myopia. Simple myopia goes upto -6D and does not extend beyond that. If there is a difference of power in both the eyes to the extent of 3D paitient usually gets double vision
by using glasses. This condition is known as anisometropia
Solution to this problem is the use of contact lenses instead of glasses. However, since you have not mentioned the power of glasses in either eye so you can take check if the aforesaid applies to your child's case.