I have encountered many here that disagree with Spanish being taught at all. Their logic seems to be that if the language is spoken by illegals, then it must be bad, and therefore it must be prohibited in our schools.
I didn’t speak English until entering public school when I was 7. I speak Spanish, English and Portuguese. All of my children are also fluent in all three, although they refuse to speak anything but English most of the time. It is a valuable asset to speak more than one language, but you are right about the cognitive benefits. Most colleges are now requiring 3-4 years of another foreign language, and many degree programs are also requiring a second language.
My #2 son learned Spanish from his friends across the street and others at school. He pursued it by dating Spanish speaking girls who were trying to learn English. Quid pro quo. His ability to speak Spanish was critical to being hired at Rubio's (his 2nd job in high school). Half the staff didn't speak any English.
This phenomenon seems to be nearly universal among "second generation" kids in the USA. No matter whether the parents' mother tongue is Spanish, Chinese, Arabic, Lativian, Thai or whatever, the kids only want to speak English!
I've seen or heard about this situation time and again from my foreign-born friends and colleagues. I think ca. 1/2 of the reason is simple rebellion against parental authority, while the other 1/2 reflects a desire not to be seen by their peers as anything other than 100% American.