Sight

'Sight' relates to vision.  It is the power of seeing (i.e., perception by the eyes).  It can also be something that is seen (e.g., What a beautiful sight.)

Site

The noun'site' refers to a piece of land (e.g., building site). As a verb('to site'), it means 'to position in a place' (e.g., I will site the slide near the swings.)

Cite

The verb 'cite' means 'to quote', 'to refer to', 'to summon to appear before a court of law' or simply 'to mention'.

Be sure your child's grammar mistakes aren't from any vision problems they may be having. Eye exams should be done at the age of 6 months, 3 years, 5 years and every year after that to ensure healthy vision.