Family-Friendly Sidescroller with Cute Marsupials

Dispell is a cute classic sidescroller in which you, an apprentice wizard are attacked by a variety of demonic marsupials--whom you have to "rescue" by zapping them with your wand, than grabbing them before they hit the ground.

It's geared for kids (hence the non-violent theme), but is challenging enough (at least if you crank the difficulty up) for adult sidescroller fans, too. And there's a co-op play mode, too, so you can play with your child, if you like. Good clean fun.

Fast-Paced Arcade-y Shooter with Online Play

Robotopia is a cheerful sidescrolling shooter with a color palette remniscent of the arcade, in which you control a robot who can fly, zapping a huge variety of opponents and bosses. The single-player game has scads of challenging levels (ten in the demo); you level up over time and can purchase new weapons and equipment, for an RPG-like experience. And once you've gotten good at the controls (see below), you can go online* and battle others in deathmatch, "capture-the-flag" or "bounty-hunting" play.