First of all, using chess.com is a necessary tool for almost all chess students. Because in 99% of families no one plays chess, and no one can become a child's opponent in practicing chess. So playing chess online is a must.
A big problem with free accounts is ads. chess.com has only minimal control over the delivery and content of advertisements. Basically it's up to Google and other advertisers. Not only are these ads distracting, but they may also be harmful to children. Removing ads is the first priority, and this requires at least Gold membership.
In addition to playing chess, another purpose of using the Internet is to solve chess problems. Five questions a day for a free account is far from enough for children who are in the growing stage. If you want unlimited puzzles, you also need Gold. So Gold is a basic necessity.
Playing chess requires review. Without review, nothing is learned. Most junior chess children do not have a private coach to help them review their games, and even if they do, it would be impossible to review all online games. This requires machine review and analysis. The free account only allows one game a day, and growing-stage students need to play at least 2-5 games a day, so they need to seek help from Platinum membership.
Diamond provides a virtual coach who points out your mistakes, gives you a chance to correct them, and evaluates your new moves. This is the function I use most often. Generally speaking, playing a game of chess can take 5 to 10 minutes, or even longer, and reviewing it should take at least 5-10 minutes. Diamond also has a function called Insight, which tells you your strengths and weaknesses. I don't fully understand utilization yet.
I always choose Diamond. My reason is: Although chess.com does not clearly explain the difference between Diamond and Platinum, some unstated restrictions may limit your use and learning. To save $40, doesn't seem worth it. This is not even worth a group lesson, let alone a private lesson. My students don’t even blink if they miss a lesson due to holidays, school events, parties, or birthdays. So why save $40 a year on your most important tool?
The above are my opinions and are for reference only.