Every child develops language skills at a different rate. Sometimes, kids who are extremely adept physically lag behind in language, and vice versa. It's nothing to be concerned about -- and at home, it actually doesn't matter that much if sentences are choppy and words aren't pronounced quite right. Parents typically know what their children are saying as if they're speaking perfectly.
In kindergarten, though, it matters. A child who can't communicate effectively to strangers is going to have trouble getting his or her point across to teachers and peers, which can leave a child on the outs not only in terms of classroom learning but also when it comes to play.
From the very start of the kindergarten year, children should be able to speak in mostly complete sentences and pronounce words correctly enough for a stranger to understand them. Effective communication is critical for real involvement in class.
And finally, a skill your child probably can't even walk in the school door without ...