We have been to WDW three years in a row in early May, and for the most part the temps have been in the mid 80's, so yes, definately bring shorts and tank tops. In the evening I would put my daughter in maybe something a little nicer - like capris and a t-shirt, but if my two little boys still wanted to wear tank tops I let them, it was fine. I guess if you were out later in the evening and you or your children are the type that gets cold easily you could possibly need a light jacket for a restaurant or if it did get chilly outside. May weather is beautiful. Have a great time.