Children who have a plus number often manifest with squint because these children use their muscles (accommodate) to correct the refractive error. Due to plus number, he can't see very small objects clearly without glasses. In order to see clearly, he has to apply extra force to his muscles and this extra force results in squint. So, once he puts on the glasses, his muscles are at rest because now the vision is being corrected by glasses. This type of squint is called Accommodative Esotropia and the standard treatment for this squint is wearing of glasses only. If squint is visible even after wearing proper glasses, only then is surgery indicated. Plastic lens (Poly carbonate) is safer and is highly recommended for children.