My daughter is in the dual language program in White Settlement ISD. She started last year as a kindergartner and will continue this year as a first grader.
It did not affect her learning last year in anything other than a positive way. They are in slightly smaller classes which can be a big beneft in learning. They are also buddied up so that the native English speaking children and the native Spanish speaking children can help each other. Also last year they were learning more of the common words such as hello, good bye, days of the week, months of the year, colors, etc. Most of the kindergartners already know this in English prior to starting school so again no hindrance there. Another advantage is that they will share a classroom with the same children all through Elementary school and will form long lasting and tight bonds with each other.
They way WSISD does it is that approximately 25% of their week is taught 100% in Spanish. That translated for my daughter into Social Studies and Science being taught wholly in Spanish. They do increase the percentage of time that is taught up to 50% as they go thru elementary school and then it drops slightly when the hit middle school.
As another poster said you can always enroll your child in kindergartner and if you feel that it is adversly affecting your child then you can withdraw from the program. However if you don't start him in kindergarten then that option is no longer available.
I personally love the dual language program and have no intentions of pulling my child out. It is a proven fact that children pick up a 2nd language much easier the younger they are. Living in Texas, speaking Spanish is definately in her favor. (BTW- neither her father nor I speak anything but English)
Hope this helps.