Here at WooCommerce i have 2 fields to choose before add to cart will be active: http://mnowak.devispace.pl/produkt/piekna-liliowa-sukienka-dla-dziewczynek/

If i will choose anything at the field "rozmiar" it will be only 1 option to choose always at the field "wiek dziecka". How to make "wiek dziecka" choosing automatically after "rozmiar" is chosen by user?

There are ids: 1st field - select#pa_rozmiar
2st field - select#pa_wiek-dziecka

So i need to do that, when user choose something at select#pa_rozmiar and if at 2nd field it will be left only on possible answer to choose i want system to automatically choose that one possible answer at select#pa_wiek-dziecka. At a default WooCommerce asks to choose at first at one select, after at second select and only after that it will be possibilty to add at cart.
screenshot of select fields