Common Core Standards
Grade 1
Language L.1.4
Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade 1 reading and content, choosing flexibly from an array of strategies.
First graders should know more words than kindergartners, but they'll still be a little lost on the meanings of some. As long as they're attempting to use their newfound skills, they'll be hitting these standards. Check the substandards for more specifics.