Is polyphasic sleeping a good idea if you're still growing?

No. Polyphasic sleep will ultimately cause the decrease production of growth hormone because polyphasic sleep results in less slow wave sleep. Hence, with decreased growth hormone production while growing, there will be less growth or better said...There will be stunting of growth. Not recommended. Get consolidated sleep to maximize physical growth.
Bad idea. Polyphasic sleep is a strategy that was developed for people who are required to remain vigilant for prolonged periods of time without having the opportunity to have a normal sleep time, such as sailing solo around the world. Children and teenagers should get their 8 - 12 hrs sleep time straight.