Flannel will be easier to push a needle through, than felt, which is nowadays made of sturdy polyester, too tightly compacted.
Use a yarn needle, which has a blunt point, and yarn, which is cheap, fits the needle, and easily obtainable.
Work in squares, for straight lines.
If your 4-year-old can successfully cut with scissors, his hands may be ready (tiny bones fused together) to manipulate sewing materials. Notice how a child's sewing kit has stiff boards? You may need to hold his fabrics taut so that he can sew, at least until he gets the hang of it.
It's wonderful, and consider it a successful beginning, whether or not the child completes the work.