This site is my favorite gaming site (since I hate all others). My username is Miss-Squirrel (on GSM), and I found this to be a really good learning experience. The free quest is fun, though pretty limiting. However, you can pay $19.95 for the premium quest, or use a promo code at gamestarmechanic.com/landing/redeem. The code is NKALCBPFZ. Beyond those sprites, you can purchase premium packs for $1.99 to $2.99. The best packs to purchase are the Underwater, Freezer, Winter Holiday, Autumn, and Mystical. (The others have boring sprites or are just repeats.) If you do decide to go with the free quest, you should consider the Gateway Pack. It grants access to Locks and Keys, which are pretty essential for good games :)
The Quests are fun, and they do have some cartoon violence, but nothing realistic. (When you die, you turn into a gear, which slowly fades away-- same for enemies you "frag.") The premium packs have challenges, which are also fun, but pretty difficult. In all, some of the challenges (you play challenges to earn sprites) are extremely hard for the average child-- either the parent should sit down with them and help them, or the child will spend 48+ hours on the Quests and Challenges. It also is a bit addictive :D
Designing games is my favorite part of GSM. You get a template of "sprites" (characters, blocks, and items) you earn in the Quests, and you drag and drop them onto a grid. You can make the level many "screens" long, or keep it at just one screen, and you can upload your own backgrounds with the premium quest. The designing procedure is pretty intuitive, but sometimes things don't exactly make sense. Here's my advice: figure it out. It usually doesn't take long. If you can't, though, look it up in the Game Alley. There are tons of good tutorial games that will facilitate your experience with GSM.
You can also "level up" by publishing reviews, reporting bugs, and playing games that were previously ignored. The bug-reporting part is a learning experience as well.
The GSM community is extremely friendly and will not usually leave hate reviews. If they do, however, you can just flag the review and the moderators will delete it.
Keep in mind that you can edit reviews but not comments. :)
I think I've provided WAY too much info here, but here's it all in a nutshell. It's intuitive, simple, a good learning experience, and A LOT of fun! If you haven't already, try it.
BTW, kids younger than six can still do it, it's just that they might need some parental help.
Good luck playing and designing! :)