Let me begin by stating that I think that there is no substitute for mental computing in the early stages of the education. I would say that prior to 12 years old no calculator should be used on a permanent basis. It may be used for the child to self-correct their homework, but by all means, not to allow him to avoid mental calculation. This is the moment to learn addition, subtraction, multiplication and division, and there will be no second opportunity to do it. Even correcting your own homework with a calculator may implant in you the idea that learning to do it by hand is a stupid thing when having calculators - and then you enter the class with the wrong foot.
Once said that, which calculator? Remember that we’re talking 12 years old, which is the beginning of secondary education in many countries. Clearly, no financial calculator is appropriated, since the children would not comprehend many of the concepts covered by the calculator.
Then, there are the cheap scientific calculators, hp10s and hp300s as an example, now in their “+” versions. None of them in Hp, and less so in other brands, have RPN as input method (let’s put aside for a moment the convenience of learning RPN or not for a child, please), and few of them have full textbook data entry (meaning you can enter an expression like you read in your maths book, and having the machine interpret and execute the whole expression)
There are some better scientific units, namely the hp35s and some casino offerings, that can be used the way described above. Many of these calculators are also programmable, and this is something children need to learn too at an early age - which is another pro for these machines.
But I think that at this age, children need to go to a graphical calculator. There are so many more things that can be done with a graphical calculator that using any other will not be accepted by y child in this mobile telephone age.
I have to confess that my knowledge of other brands (Casio, Texas Instruments) is extremely limited. I know that, in many countries, trying to replace a TI in school is a very uphill battle. But I am pro-HP in a very similar way to the tifossi in Calcio or Torcedores in Brazilian football.
The hp39gii+ is a failed effort from HP to get into the cheap graphical calculator segment. Initially designed to be the solution to the Chinese school system, both in price and specifications, and featuring a nice multi-grey, high definition screen, it was too clunky in comparison to the competition, both in and out of HP. It has not been pushed in Europe, and even not released in the US to any significant extension. It also uses a programming system semi-compatible with the hp Prime, but limited in some ways. Although in theory its firmware can be improved through its connectivity, I have not received any update since its creation.
However, it is fast, has a decent keyboard and can run off a single AAA battery if need be (for a short time: it is prepared to use 4 AAA batteries, but they are connected in parallel; hp low voltage circuits allow it to be powered th just 1.3–1.5 volts)
Then you have the hp39gs and hp40gs. If you’re in Europe, you can find them quite cheap, but you’re on your own. They are incompatible with the rest of the Hp range, and they never had the acceptance level they enjoyed in the US. So my advice would be not to waste you money here. I’dw rather get a proper HP35 than a HP39gII - the looks of the former are to die for, and you would be the coolest child in class.
Next in line is the hp50g. This calculator has been HP’s workhorse since 2006, has an extensive set of programming tools, a well defined and elegant programming language (called RPL, that no one else uses), and very good keyboard and screen quality. If you want to spend around 100€, this is the machine I would get. It allows textbook entry AND RPN, and can follow you to university (it is the most used calculator for polytechnic schools in at least Spain and France, with an extensive array of free software solutions for nearly any branch of engineering and physics)
But my clear choice (and what I’ll give my son this Christmas) would be the HP Prime. For a boy, there is no comparison: