就是=已经是， 我们在小的时候，就已经是好朋友了(就已经成为好朋友了)。 He has been my friend when I was young.

intencification 我们 从 小 就是 好 朋友 了 = we are good frings _right exactly_ from childhood.

就是 is actully two words, 就 has quite a lot of meanings, but in your example, 就 means as early as, already, long since 我们从小就是好朋友了 。 It implies "we didn't just become friends two days ago, we've been friends long since childhood." 我1小时前就做完了。I've already finished this 1 hour ago.

只是起强调作用，强调时间比较长

just emphasize, we have been friends for a long time