Term or idiom to describe a best friend since your childhood

"Lifelong friends" is about as far back as you can go because most people would understand that tiny babies don't really have "friends" so to say "lifelong", it's generally understood that as long as someone were capable of having/being a friend, they've been friends.

I am not a native speaker, but I've come across bosom friends to signify a thick friendship between people. Also bosom buddy.

One's close or closest friend; a friend one holds dear to one's heart

[The Free Dictionary]

Usage:

We have been bosom friends since we were toddlers.

A common phrase for someone you were friends with in childhood is childhood best friend

However, that can often refer to someone you haven't seen since you left school. If it's someone you're still friends with, an unambiguous phrase that is commonly used is we've been best friends since childhood.

Note the tense used in the second example. "Best friend since childhood" doesn't work as a noun-phrase, the noun-phrase is "best friend". Hence you can say "we've been best friends since childhood", meaning "we were best friends when we were children and we still are". You can't say "We are best friends since childhood", or "She is my best friend since childhood".

Shamelessly cobbled together from comments, because it's exactly what I was thinking and after nearly 24 hours no-one else has made it into an answer