To OP’s parents:
As a parent of a WaniKani student, and as a WK student myself, I highly recommend WK for a few reasons. First, mastering Japanese is going to require learning its cumbersome writing system. There’s no getting around the need to recognize and know the meaning of about 1,000 distinct characters to read about 90% of characters you would come across, and more than 2,000 to be secure. If your son/daughter is serious about learning Japanese, he/she will need a resource for learning those characters, so that he/she can access native reading materials, not just beginner textbooks.
Second, the quality of the material in WaniKani is superior to anything I’ve found (and tried) for free. The mnemonics and order of presentation are well thought-out, and the integration of character learning with vocabulary does a great job of boosting retention of both. The web software is well-maintained, and the company as a whole is run very professionally and has been around for many years. In this case, you definitely get what you pay for.
Third, learning more than 2,000 characters and more than 6,000 vocabulary words is no easy feat, nor is it very much fun after the novelty of it wears off. More than just teaching Japanese, WK trains you to be persistent, a quality very much needed for success in life, as I’m sure you’re aware. With WK, as with any spaced repetition system, consistency is key. Skip a day and you’re looking at double the work the next day. Not many kids I know have that kind of drive built-in; it was tough for me even. WK may take some amount of parental pushing, but your son/daughter will know the meaning of perseverance when he/she finishes that last level. My daughter is about halfway done after about 18 months of effort. Already, she can read almost all of the characters she comes across.
If you aren’t sure whether your son/daughter is willing to stay committed to WK, I would recommend starting out with a monthly or annual subscription and see what level of effort he/she can sustain. WK, as most systems do, lets you decide how quickly to introduce new items. While I took on the maximum possible, which probably averaged out to about 20 items per day, learning and reviewing took hours of my time every day, at precise review times, for more than a year. For a full-time student, I think a more reasonable pace during the school year, one we set for our daughter, is 40 items per week: 20 on Fridays and 20 on Saturdays. That amounts to about a level every 3-5 weeks, I think. (There are 60 levels total.) During summer and winter break, we usually require 15 every day, 7 days a week. If your son/daughter can maintain this pace over a few months, you might consider upgrading to a lifetime membership to save you in the long run, along with getting a commitment out of him/her to finish the whole program, of course!
Sorry for the long article, but I hope I have given you the right information to help you make your decision.
(edited to fix my math!)