Our neighbours are what one would call a really truly multilingual family. The mother is Swedish, the father is Italian. Together they speak English. The kids speak Swedish with their mother, Italian with their father. They lived the first years of their life in Switzerland, which means the children speak "Swiss German" (believe me, it's as far from German as Chinese from English) , They now live in Germany, so everyone is learning "High German" (Hochdeutsch, the "clean" German), and the kids are in an international school where English is the primary language, practicing the English they've picked up from their parents. In short, you have 3 kids aged 8, 6 and 3 who understand and speak 4 languages, 4 languages that their parents don't even all master.
Apart from the amazing-ness of it, I found it to be truly encouraging, to see that kids that young could simply pick up all those languages and feel at ease in any of them. We had coffee one afternoon and the kids indifferently spoke German or English with us, while conversing with their parents in Italian and Swedish.
I can't wait to ask my daughter questions about what it is like to learn more than one language at once. Although I now speak and understand four foreign languages myself, albeit at different levels, I learned them once my French was already there, meaning that gender, concepts and thought structure were already shaped in my mother tongue. I am especially interested in trying to understand what it feels like having different words for a same "thing" especially when those words convey different impressions or feelings, such as different genders for example. It'll be interesting, for sure.