What an awesome idea! I run the voluntary program in my school district for instructing elementary-aged children in foreign languages (French, German, Spanish). For the younger kids (grades 1 &2), I like to use a lot of manipulatives, physical activities and music to teach vocabulary because they are not really ready to read a second language. There's a lot of great stuff available in libraries and bookstores for teaching Spanish. Less for French and very little for German. Sometimes, you have to get creative. I often get Spanish workbooks and then change the vocabulary to French for my students.
Since you don't speak a second language yourself, this is the perfect opportunity to learn along with your children. I looked into the PowerGlide programs but they didn't suit my teaching style. However, they may be just what you need. The languages are taught as a part of an ongoing adventure and slowly introduce vocabulary. The lessons combine audio CD and workbooks. I've seen the materials available on eBay several times.
Another fun thing to do is to switch the audio on a DVD that they're familiar with so that they're hearing it in the chosen language instead of English. It can be done by choosing "setup" from the main menu and then changing the language. I use it in my French classes for some of my Disney favorites, especially those that are French (Cinderella, Beauty & the Beast, Hunchback). If the kids can relate to it, they'll pick up the words quicker. For older children (3rd, 4th & 5th grade), I also have the subtitles running in French so that they can read what's being spoken and pick out words that they know. The added plus is that the songs and dialog are done by native speakers, so that the children not only learn the words & pronunciations, they also become familiar with the inflections and cadence of the spoken language. We Americans tend to speak second languages with our flat accents rather than copying the rhythm and flow of words that a native speaker would use. It really sets us apart when traveling.
I could go on & on with the games and songs that we do in my classes but I'll let others offer their suggestions. If you have questions, feel free to PM me.