Carhartt Kids' Long Sleeve Flannel Button-Front Hooded Shirt

Is the whole shirt fleece?

No, just the hood. The body is 100% cotton yarn-dyed plaid flannel; the hood is a heavier cotton-poly fleece. You get flannel comfort with a warmer hood.

Will it last long enough to hand down?

That's the design intent. Yarn-dyed flannel holds its color rather than fading like a printed plaid, and the stitched-on pocket label is Carhartt's mark that it's built to be passed along.