Would a Spanish immersion program be beneficial in ...

I think that it is an excellent idea to immerse a child into a Spanish and English educational program at an early age. Children learn languages best at a young age and there have been many studies that show that bilingual children tend to have higher IQs. In addition, proficiency in both languages comes in very handy in the world of employment in the United States today. 

The high school at which I was principal offered a very successful French Immersion program. I would strongly recommend a language immersion program. Research shows that for average to above average students there are no long term negatives in learning English and the best place to start is in the earliest grades. The benefits to employment prospects when you speak 2 languages are obvious. I would not agree with the comment made regarding IQ. I have seen that data too but in my opinion it probably just reflects a natural weeding out of less able students from the program rather than a real increase in IQ.