I seriously thought this post was a joke at first! Mama, do NOT worry about a kid that young not doing letters or numbers yet. That is a very complex skill and some Plain. Aren't. Ready.
If this was a seven or eight year old? I'd be more concerned. But the multiple steps of understanding the quantity or value, the motions of tracing or copying the strokes, AND pencil grip? It's more unusual your other kids were all ready at the beginning of kinder age than that one kid wasn't, truthfully.
I'd be working on basic pencil skills - lines left to right, finger tracing shapes in shaving cream or sand, lots of clay and hand putty work, mazes, basic cutting and pasting activities, lacing toys, beading, etc. Set aside more formal handwriting for a year and try again later.
My son was in this exact position last year. It turns out he just had less fine motor control than his sisters at the same age and the work was too specific and demanding for his maturity. We backed off and did a lot more of the readiness activities I listed above and tried again half a year later. All the sudden he COULD do it and with much less adieu, but his confidence benefitted so much from tracing and mazes and fun activities working with strokes than me bludgeoning him with a workbook he wasn't developmentally ready to handle.
Edited by Arctic Mama, 11 November 2016 - 08:01 PM.