Remember Dr. Ivan Pavlov (1849-1936) and his experiments with dogs? He is the father of the conditioned reflex which is an action or feeling that we have in response to a specific situation or stimulus. When he fed a dog, he sounded a bell at the same time, so that the animal associated the food with the sound. When the dog heard the bell, its mouth watered.
People are equipped at birth with all the neurological and anatomical possibilities to master a language, or two, or three, and as Aristotle said: "All men by nature desire to know" and the first thing they want to know and master is language itself. A child immediately associates a sound with an object, whether that sound is table, mesa, tavolo, taulukko, masa,Tisch, taula... whatever. No problem at all.
The problem may start when the child's father speaks, say, English, and the mother Spanish. The family may live in Argentina or in the US. This is when Dr. Pavlov ties in with all this. Naturally, living in Pennsylvania, Scranton, for example, the little one will speak English, not only because his father does, but because everybody else does too. From day one the baby will associate the sounds of Spanish with its mother who will address, lull, sing, coo, lullaby the infant in Spanish... always. Likewise, the child will identify the father with the English language.
In the U.S. I wanted my two daughters to speak Spanish fluently and I never used English with them. When the three of us moved to Madrid I informed them that we were going to change languages: "from now on we will speak English to one another." A lot of water has gone under the bridge since that day but we have never, and I mean never, exchanged a single word in Spanish. They are bilingual and speak what I would call educated English and educated Spanish.
Lorraine, my eldest, once said that if we spoke Spanish to each other it would sound hollow, artificial, strange, weird, odd, out of place. Imagine how conditioned we are! We have been through some rough times and we have had our ups and downs, bickers, spats... but always in English.
It is not easy and I cannot lie to you. You will need a lot of persistence, discipline and be ready to expose yourself to criticism. Monolinguists have a certain animosity, a love-hate attitude towards foreign languages and are prone to reprimand and browbeat you and your child as being different or not belonging to the group. Do not mind them. Try not to, at best. In the long run your child will grow up with two important communication tools with which she will be able to cope better with the slings and arrows of fortune.
Just think how hard it is for the monolingual grownup who strives hard to acquire a foreign language and how easy and simple it is for the child. Do not deprive your offspring of this tool, weapon even, that will keep them in good stead as long as they live. Temptation to switch languages will lurk in the shadows of your mind and call to you... keep it at arm's length, do not listen, never relent; plug your ears and keep at the helm of the target language, on course, steady as it goes.