I've used Rosetta Stone for Italian. I only got through the first unit before deciding I wasn't going to have time to use it and returned it. It is quite time-intensive. I'll probably buy it again someday, but at the time I couldn't justify that much money for something that was going to sit on the shelf for who knows how long. It's very thorough and I found that a couple of years later I still have very vivid memory of the vocab I learned.
One of the nice things is it teaches you to speak without translating. For example, rather than telling you that "apple" is "mela" it shows you a picture of an apple and says the word mela... says it out loud and/ or shows in written on screen. The headset is for checking your pronunciation, but you could all look at the screen and use it together if you don't do that part. You do have to type the answers to questions in some parts, so if you were all doing it together you'd have to take turns answering or all agree on the answer, etc.
One of the bad things (for me at least) is that it doesn't set out and explain the rules of grammar. So you learn how to say sentences properly, but you never learn the rules that govern why sentences are structured the way they are. This works when you learn a first language as a baby, but I think for learning a second language, it's hard to actually be exposed to enough properly written or spoken language to really learn the rules just by listening or reading. I think it would work best combined with some traditional workbooks that explain the rules- though maybe you'd introduce that once your kids are a little older.
I also really like the Pimsleur language program, although it also would probably be best combined with other methods and more vocabulary. I think it would also be suited to older kids or adults, just due to the subject matter in the lessons (traveling driving, converting currency, etc.) which might not hold the interest of 6 and 8 year olds.
