I play with my 4 and 6 year olds.
Mario Kart is perfect, as there's a really good autopilot mode you can put them on that basically keeps them on the road at all times.
Guacamelee is good as you can 'pull' player 2 along as you progress with difficult jumps etc. My kids loved being involved in the fighting bits. It's also got bright colours and sounds which engaged them.
Mario Party is good too, both mine can play about 75% of the minigames without any real help. It's also really cute and they love the board game idea.