Parent and child product display

[TAIWAR]

VM266

I've create a product parent and child poduct and I use stock quantity.
all my child prodct have a stock quantity but not the parent.
On frontend, when I have the product detail I haven't got the button "add to card," in fact I must select the child product then I have to button add to cart.
I think that I must configurate something else to display this button. So I want to know how can I allow the button "add to card" for all parents product even if the stock quantity is 0 ?
thank you