Is there purpose in your elementary or middles schooled aged child in taking Spanish when they will probably take it in high school and college? This is a good question, and deserves a good answer. Because, any language is far too complicated to truly understand and be able to use well with only high school, and perhaps even college, study. Also, younger children are like sponges and just love to learn. Have you noticed ironic things your child knows or copies someone doing? They are craving more knowledge! Spanish is a wonderful thing that will be useful to them their entire lives, even if it is just vocabulary they learned in 3rd grade.
Not only that but it's the second most widely spoken language in the US, second only to English. AND it's the second most spoken language in the world (Next to Mandarin)! If your children can speak Spanish, they can communicate with over 70% of the world's population!
Also, have you ever tried to learn a language after your brain stopped growing at the rapid pace it was when you were a child? It takes lots of hard work, and you still might never get fluent without an accent. But, think about how fast missionary kids learn another language, much faster than their adult counterparts! Why is this? Because their brains are ready for it, longing for more information, more knowledge! Why wait till high school where they could struggle through it and still only have about a 5th grader's level when they could learn it easily now, and just build to a higher level easier later?
I was a missionary kid, and learned Spanish at the same time as English, going to Chilean public schools through 3rd grade (except PreK in Costa Rica) I learned to read and write Spanish before English. Why? Because Spanish is far easier! Have your children complained about how many rules English has, and how many exceptions? This is not the case with Spanish. You could list all the Spanish rules on one hand, and there are practically no exceptions!
Spanish also has many similarities to English (and Latin which the both derived from). It uses the same alphabet, unlike many other languages such as Japanese, Hindi, and Russian. It also has very similar pronunciations for it's letters, unlike French and German.
So in a nutshell, why wouldn't you want your young child to learn something that will be so useful to them, and they will eventually want to learn anyways? Why wait till it's hard, when you can do it now while it's easy? And why miss out on having a homeschool missionary kid with a native accent teach your child Spanish, at a great price?