Have you had a look at "The Easy Spanish"?
It can be tailored down quite easily for the younger member, whilst the older one gets more 'meat'.
http://www.theeasyspanish.com/
reviews: http://www.theeasyspanish.com/reviews.htm
Personally, it's the better program I've found, but it's not under $100, sorry.I'm using it with my 8 and 11 yr old children.
The other books I've used for Spanish is the "Teach Me" series:
http://books.google.com.au/books?id=u-DFX0T8oGAC&printsec=frontcover&dq=teach+me+more+spanish&source=bl&ots=dk_CPrgIc5&sig=Mkz6FDKM14JkBHkRqA0249SGPLM&hl=en&ei=ut0gTOr9MtOkcf-fgCk&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAsQ6AEwAA#v=onepage&q&f=false
These are cheaper, but we found the songs to difficult to learn. For younger children, the colouring pages are great. The teacher's book does have some of the vocab., but I wouldn't rate this series as high as "The Easy Spanish".
Another program I've heard of (and seen for FRENCH) is from Galore Park.
http://www.galorepark.co.uk/product/parents/46/so-you-really-want-to-learn-spanish-book-1.html
Also looks very good, broken into managable lessons, but the site says Spanish is a little harder than the French.