The Pre-K program in most counties was started as an extension of the Head Start program (federal to assist poverty level children who did not have same advantages as children whose parents could afford to put them into private programs). At this point it is limited because of space issues, not enough certificated teachers, and
funding issues.
You can really tell in Kindergarten which kids have had PreK and which have not. The children who have been in Pre-K move through the curriculum faster because of prior exposure. For the most part, the children are all pretty much at the same level by 2nd or 3rd grade depending upon individual ability levels. We do get children in kindergarten who do not know how to spell their name, do not know colors, do not know the alphabet, etc., in PG County schools (and I'm at a really good one!). The ones that I really feel badly for are the ones whose primary language is something other than English. Those poor babies come into school at age 4, not knowing anything about school and not being able to communicate in the same language -- many tears are shed the first few weeks. ESOL (English for Speakers of Other Languages) services don't start until Kdg. Makes me crazy!
My four kids never went to PreK or to a daycare center. We had home daycare for them & made sure that they all knew the alphabet, how to write it, how to write their name, colors, etc. before they went to school.
As long as you spend time with your children, explain as many things as you can in answer to their questions (why are the leaves green?), and READ to them, they'll be okay.
Personally, I think the homework bit starts too early and we're piling too much on our kids. They need more time to just be kids. And that's coming from 27+ years in the teaching field. Today's high-stakes testing and the demands of the curriculums to meet the federal legislation (No Child Left Behind) are just ridiculous. The idea is good, the implementation sucks! We spend so much time testing, there's little time to teach and we're stressing the kids out like you wouldn't believe!