The Universal Prekindergarten program is wonderful and most districts have it. It is not full day but it is 5 days for 1/2 day which might help and transportation is provided. It is paid for through those of us who pay their taxes so it would not cost you any additional money.
The only problem is that there is limited space. It was originally designed for people in situations like you describe - you want your son to go to preschool but the money is just not there!
When I was a teacher in the program, at its inception, families who were in need (need to supply your W2's) were given the slots first. Once all interested families were taken care of than the rest of the community (those who made more money)could register and if need be a lottery system would be held.
In my area everyone is put into the lottery system regardless of income. So if you are interested you need to call your school district and first find out if they have it and second find out when to register and be screened etc. You said your area doesn't have preschool, but I would call the school district just in case because this program is not widely publicized. Good luck.
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