As with every iteration of Tetris, the basic concept here remains the same - differently shaped blocks fall down the screen, and it's up to you to position them so they create a full line, without any gaps. Do this, and the line will disappear.
Featured here are a mixture of single player, co-op and versus modes, with support for up to four players either online, or locally on the same console. Whether you're going it alone in Marathon mode trying to clear enough lines to reach level 15, facing off against a sibling or rival in Battle Ultimate mode (where you can even call on power-ups to give you a helping hand), or at least trying to clear *some* lines in four player co-op Tetris, where you play across a wide stage, with each player taking control of a separate chunk of the grid (which is every bit as chaotic as it sounds), there's a decent array of modes here - especially considering the budget price.
In terms of accessibility here, there's little that younger players might struggle with. With nothing in the way of reading beyond a brief objective "Play for 3 minutes" or "Clear 40 lines", all that your child will really have to be able to do is arrange the shapes to clear lines. As a game with near universal appeal to young and old, chances are they'll be fine!