About “how we support children’s communication in bilingual and multilingual settings”, in my opinion, there are four methonds we can do:
Method 1: talk to babies in a way that helps them learn faster;
Method 2: create a language-rich environment for babies;
Method 3: consistently expose babies to the correct use of the language in many different linguistic contexts;
Method 4: encourage babies to actively engage in interaction

You can read more details in
▼ How to help babies acquire languages?
How to help babies acquire languages? (Method 1)
How to help babies acquire languages? (Method 2)
How to help babies acquire languages? (Method 3)
How to help babies acquire languages? (Method 4)

As to “why why we suppoort children’s communication in billuigual and multilingual settings”, the answer is much simple: being bilingual or multilingual helps children become more intelligent, such as superior performance on concept formation tasks, enhanced ability to restructure perceptual solutions, stronger performances in rule discovery tasks, greater verbal ability and verbal intelligence, precocious levels of divergent thinking and creativity, and higher levels of cognitive development at an earlier age reached by bilingual children than their monolingual peers. See more details in Bilingualism and cognitive development.

Hope this helps!