My name is Johnny, and I’m an English Language and Literature major at Oranim College in Israel. During my Language Acquisition Course, I have encountered several issues regarding this matter, including one which is similar to that which you reffered to and I would like to point out several points:
To begin with, it is possible to raise a child as bilingual, do not worry. It might be a proccess in which the child acquires both languages at the same time (parents are native speakers of different languages) and the child is raised bilingually. It might also be a proccess in which a child acquires only one language up to a certain point and then acquires another language (for example, immigrants’ child who speaks Arabic and at the age of three years old moves to Norway). If the child is at an older age, it does not necessarily mean that it would be easier for one to acquire second language.
The difference between these possibilities is that in the first case the child acquires both languages simultaneously, thus creating a database of concepts and language knowledge in both languages at the same time. The child has to comprehend different syntax, pronunciation and sometimes different vocabulary concepts (for words which may exist in one language but lack in another) so the process might seem longer or more difficult but if you compare it to a monolingual child, the steps are just the same. In the next case, the child might already have concepts of objects in his life in the native tongue, and only then he begins to utilize these concepts into a new language which he acquires, for example in his new school in a new country. Since he builds his knowledge upon another language, it may lead to mistakes which are totally natural such as mixed word order and such, but yet again, even monolingual children do that when they meet more complex language as they grow up. There is a third case in which (for example, the parents) the child already has a solid comprehension of the language and then he uses it to acquire the new one, usually by translation.
To simplify, a multilingual child may acquire both languages at the same time and build two seperate language systems. One may acquire one language and build the second one upon concepts already known from the first one, but still acquire both of them (in elementary school for example) .Another things that might happen is that the child grows up with a native tongue fully comprehended already and then builds a new language system upon it (usually the parents).
My bottom line is that children may acquire two (or more) language in a different phase of their lives, yet their mastery over the languages might not be different than a monolingual child who goes through the same stages and usually does the same types of mistakes in the process. People might think that the multilingual child knows fewer vocabulary items in a certain language, but do not forget that he might know different concepts in each language or the same concepts in both languages, thus his vocabulary knowledge might be similar or even larger than that of a monolingual child.
Of course, as said here before, the best way to help the child (both monolingual and multilingual) is by exposure. Meaning that as long as you as parent expose your child at home to your native tongue and the environment would expose the child to another language constantly, the child would acquire both of them. Regarding the mixing, it is totally natural and sometimes even help the child in comprehension of a language if the mixing serves a purpose (for example: two words which sound the same and make the child confuse the two, then the parent can understand where the mistake comes from and correct it).
Warm greetings from Israel!