two of my brother's sons were only spoken to in Spanish by their parents until they were about 4... (my brother's first language is English, but he can speak Spanish, his wife's first language is Spanish, but she can speak English)... both boys are now fluent in both languages... it did not take them long to pick up English... believe me, the rest of us in the family were concerned that the boys were only going to know Spanish... my brother kepting telling us not to worry about it... it worked out in the end...
I raised my kids speaking only spanish to them for the first 5 years of their lives and started them on English once they started school. They were speaking English within weeks.
My son speaks to my granddaughter (age 2+) in English and my daughter-in-law speaks in Spanish. My granddaughter speaks mostly English, some Spanish, but understands the Spanish. It is interesting to see how this works. She has opportunities to speak Spanish, but mostly uses English.
Children pick up languages very quickly, and if they grow up bilingual, it makes it easier for them to learn others as well later in life. That said, Klingon would be one of the more useless languages to be multilingual in.
In a mixed language community (e.g. late 1800s Hawaiian pineapple plantations) the children entertain each other through the day while their parents work. Ordinarily they create a CREOLE ~ which is a totally new language with a wider vocabulary than any single contributing language but which usually has a grammar close to that of one of the major contributors.
My understanding is that Hawaiian Creole is based on Japanese, but has an incredibly varied vocabulary. English is a Creole based on Old West Saxon ~ kinda ~ which has morphed into a totally different sort of language than those found in Europe. English, BTW, has the largest vocabulary on Earth.