Getting Started With Spanish by William E. Linney and Antonio L. Orta is a fantastic resource of 177 lessons of mostly grammar that take less than five minutes per day. It’s mastery based, and I believe could be appropriate for children as young as kindergarten. The author provides free pronunciation resources at www.gettingstartedwithspanish.com.
You can typically buy very inexpensive flash cards to develop vocabulary at your local educational store, and I personally am interested in The Complete Book of Starter Spanish and The Complete Book of Spanish for my young children.
Free Spanish Readers For Kindle:
An Elementary Spanish Reader, by Earl Stanley Harrison
A First Spanish Reader, by Alfred Remy