Not really. What would really be "letting down young boys and girls" are parents who allow entertainment to substitute for the hard work of teaching values to their children. If any parent feels strongly enough about a particular value, they will take time to pass that value on to their children and ensure they've understood it. If that is done, then the children should not be so easily swayed away from that value by a 2 hour bit of entertainment. Indeed, if the lesson is well-learned, the children will raise the value conflict on their own (if they're too young, the parents are free to point out the conflict). However, the filmmakers have NO OBLIGATION to produce something that won't "let down" any particular "young boy or girl". No obligation whatsoever. We, as the audience, are NOT entitled to be satisfied on that score--ever. We merely have the right to agree or disagree with what we see and say so.
How about if it's EVERY piece of 2 hours entertainment they're likely to see? Because the latest study suggests that girls might as well give up on a equal chance at a career alongside men and aspire to be moms and girlfriends instead. Don't bother going into acting either girls because you're twice as likely to be unemployed even if you are willing to flash your push up bra on screen. This attitude doesn't surprise me but it's still wrong.
And nobody is suggesting that every movie has to have a 50/50 gender divide but a franchise like Star Trek is a golden opportunity to do so and they screwed it up big.