There are many ways to help your child develop his reading skills. Sight words can be practiced using flashcards, which you can easily make at home using index cards.
The problems you describe could be the initial clues that this child might have learning disabilities. The fact that he needed language therapy adds to the possibility of this diagnosis.
Most children reverse letters and numbers until about five or six, some longer. I would not be worried. Watch and see if this pattern disappears by the time she finishes kindergarten.
It is not uncommon for three year olds to not recognize letters and numbers. I suggest that you hold off on these exercises for now and just have fun with her.
Under the public law, IDEA, every public school program must have a Child Find program to evaluate children ages 3 to 5 who appear to be delayed in any area of development prior to entering kinderg
Your description of your four-year-old son is complex. I recommend that you meet with the preschool evaluation service of your neighborhood public school.