B.C.
I bought Rosetta Stone used on Craig's list and saved 75% to 80%. Its hard to raise children bilingual if they have no one to be bilingual with. I would suggest you wait until 9th or 10th grade and then have them learn the language. I did that with German and emersed myself in the German in Germany. That was 40 years ago and I still remember some. I can hear and translate the war movies, but cannot speak very well because there is no one around to speak with. Although I was at Disneyland several years ago and was able to assist some German tourists find their way to Frontierland and the Golden Horse Shoe Review.
I'm sure you already know there is more difference between Mexican and Spanish than there is between British English and American English. So choose which one you want your kids to learn.
Good luck to you and yours.