Agree with others that playing is what it's all about, for quite some time yet.
I gave my son his first bow when he was 20mnths old. I made it from a maple shoot. VERY easy to do. It measures 33" and draws about 5lbs at 14" -if I remember right. A couple Surgeon's Loop knots at the end of some cordage serves as a string. I used ~3strands of B50.
The arrows are 3/16 doweling, using black soft plastic "chair tips" from a hardware store for blunt points. I also used .22 casings for blunts as they fit on the 3/16" doweling perfectly.
I got a little fancy with his first arrows, staining them in rich black walnut, fletched them with grouse tail feathers, and made fluorescent wraps above the self nocks. Well.. he launched em into the great yonder in no time.
So I went to ugly but functional fluorescent blaze spray paint and less labor intensive fletching.