New Super Luigi U is an expansion pack for New Super Mario Bros U, available either as a download, or as a disc based release. If you choose the download version, you'll need a copy of New Super Mario Bros U in order to play, while the disc based version can run on its own.
Following a similar format to the usual 2D Mario platform game, New Super Luigi U is a collection of over 80 levels, where the only real objective is to get to the end. With obstacles and enemies to avoid, three hidden gold coins to (optionally) collect in each level, and tiny platforms to navigate without falling to your doom, New Super Luigi U has been designed to be considerably harder than the Mario games that have come before it, and is aimed firmly at hardcore Mario veterans. With Luigi controlling like he's been dipped in vaseline before being released into the levels, slipping and sliding all over the place (usually into enemies or off a cliff), and a 100 second time limit meaning you can't afford to make that many mistakes, your child will have to have a pretty much perfect run to make it through each level, which may frustrate some younger children. With some precise platforming required, it's easier said than done.
While the levels have been designed to be incredibly tricky, there is a slight concession in the game's multiplayer mode, which lets you play as a character called Nabbit. Without Luigi's awful skidding, and with the added advantage of being invulnerable, Nabbit can simply run through enemies and objects without being harmed. He can still die should you fall into a pit or get squished, but he certainly makes the game a lot easier - which is why it's a shame only one person can play as him. If you're playing in single player, there's actually no option to play as anyone but Luigi - but there is a handy cheat. If you want to play as Nabbit, all you have to do is hold the B Button on the Wii Remote, or the ZL button on a Pro Controller to play as the invincible one, making an incredibly solid game that much easier.
It's also worth bearing in mind that you'll require a Wii Remote, or a Wii Pro Controller to play New Super Luigi U, as somewhat inexplicably, you can't play using the GamePad alone.
As a Mario game, there's little for parents to be concerned about here, with no violence, gore, sex, or swearing. You may find yourself uttering a few curse words under your breath, mind you.