Generally speaking, I find that woodwind instruments are easier, as they do not require harmonics, only one melodic line. That is why the recorder is the first instrument for many kids. On the other hand, kids under the age of 10 might find it hard to produce and sustain a steady sound on a woodwind instrument, as controlling one's breathing and placing one's fingers precisely require some coordination. So a keyboard comes in handy for younger kids, because producing a sound is easier.
Some have found that very small kids can learn the violin .
Bottom line - woodwinds, except if you are 8 years old...