Children can become bilinguals from the beginning if they are exposed to both languages sufficiently. The good news is that children acquire languages naturally. You don’t have to teach them; just interacting with them in a language will allow them to acquire it. The bad news is that children also forget a language quickly if they are not exposed to it anymore. To be safe, you should keep using the language with your children at least until after puberty. Eventually, whatever language you chose to use with your children will become the language of your relationship. It would feel really odd to start using a different language.