I just answered a similar question here http://answers.worldnomads.com/questions/japan/11613/is-it-safe-to-travel-to-japan-with-a-16monthold-baby-in-march--april-2012-
which you'll find useful.
radiation levels in Tokyo are back at background level (and have been so except for a couple of days in March 2011). Some people are concerned abut contaminated food, but apart from a few, rare cases (and with products since removed) there seems little to worry about. The major concern is if - again IF - there's elevated levels of cesium or iodine in the food chain, they may have an impact in the long term. Short term stays by tourists are not a significant concern.
Phil at the safety hub about 6 years ago