I think the idea of a nanny or trading time with a spanish-speaking mom would be the best route. Granted, you have to trust the person whom you will leave your children with whole-heartedly, which is really hard if you just met them.
I say, since you're on a budget and might not trust some stranger who happens to speak spanish with your kids - - get the books that are printed with both Eng and Span on the page from the library, watch the dvds, turn on the spanish cartoons for half an hour a day, get the spanish versions of games you play.... I have heard wonderful things about Immersion camps. Some are for 2 months during the summer for older kids. Not sure what they have for the younger kiddos like yours, but look into that b/c I have heard such wonderful things about it.
See if your 6 yr old's school has a dual-language classroom. He will get a certain portion of his day/classes in English and a certain portion in Spanish. 50% of the students will be spanish-speaking and 50% english-speaking.
And if you really wanted to, order some old spanish text-books off of Amazon.com and teach yourself and kids at the same time. Search for "Spanish Textbooks for elementary school" "...middle school" "...high school" and there are lots of choices. Unfortunately, the book I used to learn Spanish isn't on Amazon.com anymore, so can't help ya there.