A: It depends on the child and the instrument. Average age to start violin or piano is 5-7; it’s possible to start younger, but there are special considerations such as instrument size and a teacher who specializes in working with very small children. Average age to start guitar or mandolin is 6-8 due to size and coordination issues. If you have a very small guitar (such as a guitalele — a 6-string ukulele which is tuned the same as a guitar) and if the child has a strong desire to play and good concentration levels (for that age), then it may be possible to start younger. (See the Teaching Manual for an in-depth discussion of this subject.)
A: The instrument that the child is the most interested in is usually the one he/she will be most successful with. If the child has no preference, the piano/keyboard might be a good choice; it’s a good basic instrument and students learn both the treble clef and the bass clef. However, if you happen to have a guitar, mandolin, violin/fiddle, or ukulele laying around at your house, any of those instruments would be fine to start with as long as the child is okay with the choice and the instrument is not too large for the child to handle.
A: Absolutely. Just use different color tabs/flags for each child to mark starting pages. The only book that each child needs his/her own personal copy of is the Theory Workbook because the student writes in that book and even composes his/her own songs.