The level and scope of vocabulary among bilingual toddlers may not be different from that of monolingual toddlers. But bilingual infants spent a lot of time trying to encode the language the words belong to. According to researcher Traute Taeschner, the capacity of a bilingual child to produce new words is split between the two languages he is exposed to. If toddlers are exposed to two languages simultaneously, they will have two systems whereby they develop words in each of the languages for the same objects. Therefore, a bilingual toddler will give a priority to new words and avoid other words that are alike (See Reference 2, pg. 56 & 57).
A bilingual toddler makes use of words from the two languages he has been exposed to. In some cases, the child blends words from the two languages. But according to researcher Francois Grosjean, it's only children between ages of 2 to 4 that mix vocabulary during first-word stage. Consequently, bilingual toddlers learn words from two languages in early stages of their development. Just like monolingual toddlers, bilingual toddlers begin by producing single-word sentences before progressing to multi-word sentences. But as compared to their monolingual counterparts, bilingual infants face the problem of discerning meanings of words across the two languages they have been exposed to (See Reference 2, pg. 55 & 56).
Children aged 2 and 3 start to make longer utterances based on the rules of syntax of the first language. Bilingual toddlers learn the syntax rules and grammar of their first language and apply it in the second language. Longer sentences produced by a bilingual toddler may have vocabulary from the two languages in one utterance. In some situations, they mix rules of grammar of the two languages they have been exposed to and further use syntax that is considered correct in both languages.
Code mixing is the unconscious mixing of two languages. Code mixing in toddlers can be interpreted as lack of ability to distinguish two languages. According to Jan Petersen's study in bilingualism, toddlers mix languages because they are overburdened with the process of language acquisition. Consequently, the majority of mixings are lexical even though they may occur in other levels of language development.