Undercoat,
Good to see you supporting your kid in what should be "an adventure," for the whole family.
It'd help if I knew which models of the Jupiter and Yamaha you've lokked at. Also how old is your child? Not all Altos will be the same "size." By which I mean, that some are easier on small hands than others.
I have a 10 year old son who plays Alto. We started with a rental saxophone. This was a "Lachlan" Alto. Whilst it was a solid horn with a "decent" sound, the keywork and layout are lousy.
I am 6' 3" with large hands and long fingers. I cannot play the bottom notes on this Lachlan with any fluency, due to the fact that I can't reach the approprite keys without taking my other fingers off the sax. My 10 year old son never stood a chance.
There are many different "student" horns like the Lachlan. Some are better than others but you really have to know your saxophones and get a decent player to help you if you want to go down that path. The same can be said for buying a vintage saxophone.
I have not owned a Jupiter, but have played a Jupiter Tenor sax in my younger days. I recall it as being a good student sax. Many of the other forum folks will be able to give a much more detailed commentary on the Jupiter. They are a very common sax.
I have recently sold a Yamaha Pro Tenor sax. I owned it for over 14 years. I have also played most of the Yamaha models both student and professional. For your child, a Yamaha would be a very sensible chioce.To generalise a little, Yamaha saxophones have a well earned reputation for the following:
*Well made, with good lacquer and solid construction.
*Very ergonomic key layout. They are easy to play.
*They play in tune. Not all saxophones are easy to play in tune.
*They tolerate a wide variety of mouthpieces.
*They have a good sound. Yhey blend well with the school band. HEAPS of school kids play Yamaha saxophones. Many Bandmasters insist students play a Yamaha.
*They hold their value well and are not difficult to sell, if and when your child graduates to a professional model saxophone.
*Should you back over it in the car, most capable repair persons will be able to readily source parts for a Yamaha.
Many of the same points could also be made in favour of the Jupiter. By all means listen to the opinions and experiences of others here and consult a saxophone teacher/player and get their advice too.
Last but not least, if at all possible, let your child play as many of the candidates as possible and see that the child can at least reach all the keys comfortably.
Finally, don't hesitate to keep asking questions. Good luck.