I like WordGirl's Word of the Month for May: Challenge, because I think it's good for kids (boys and girls) to tackle things that they find challenging. Challenging is actually one of my three-year-old's first big words. She was doing well with puzzles, so I bought her a couple that I told her were "more challenging". And she totally got the concept (though she tends to use "it's too challenging" as an excuse to ask for help now).
But Scholastic and PBS Kids have chosen Challenge for the word of the month because WordGirl is the official ambassador for Scholastic's Summer Challenge encouraging reading. Kids can log their reading minutes and win prizes. Scholastic posts stats on minutes spent reading by school, as well as collectively across all participants.
But however you look at it, Challenge is the word of the month for May.