“Holy infant so tender and mild”
As I kid I didn’t understand the word “mild” being used to describe a baby. I’d only heard it in the context of salsa. So infants come in both mild and spicy verities? But either way, apparently their meat is very tender. 
Most of the infants I’ve known have definitely been the spicy variety.
Out jumps good old Santa Clooves…
The dog nearly ate the baby Jesus in the nativity scene one year. Everyone ran around yelling “Lily’s got the baby Jesus, Lily’s gonna eat the baby Jesus, drop it Lily!” It was hilarious and we all wondered what the “tender and mild” baby Jesus must taste like. In my mind, he would be sweet, sugary, and delicious - like a chewy marshmallow.
I thought “good King Wences last looked down on the feet so even.” I assumed he died after that last look. My dad used to sing “Deck the halls with Boston Charlie, Walla walla wash and kalamazoo.” We didn’t stand a chance.
For me it was “I Saw Three Ships Come Sailing In.” Growing up in North Dakota, where it always snowed at Halloween, and never having been to any coast, I couldn’t imagine ships moving on Christmas Day unless they were icebreakers.
“There’s no such thing as a tough child. If you parboil them first for seven hours, they always come out tender.”
—William Claude Dukenfield
I actually realized I didn’t know for sure the meaning of “God Rest Ye Merry Gentlemen.” So I looked it up. Turns out, it’s actually “God Rest Ye Merry, Gentlemen.” The first part is an older form of “rest assured.” So the lyrics mean something like this:
“Rest assured, guys. Don’t be sad. Remember that Jesus’ Birthday is coming. He was born to save us from Satan, even before we were Christians! Isn’t that joyful, comforting news?”
Last year (I think), I decided to look up the meaning of the German words I’d been singing since high school choir. As loose as the translation is, I think it could have been fixed if the translator wanted to.
The literal translation is given, followed by my attempt at a more natural translation (but not worrying about it fitting into the music).
Silent night, holy night.
All are sleeping. Alone watches
Only the close, holy couple.
Blessed Boy with curly hair.
Sleep in heavenly peace
Sleep in heavenly peace.
My translation: On a silent, holy night, while everyone sleeps, A holy couple gather close and alone watch the blessed boy with curly hair as he sleeps in heavenly peace.
As for curly hair, I can’t find it now, but I read then that curly hair was associated with a loving, kind temperament in that time. But I’ve more recently read that it may just be how Jesus was typically depicted in a nativity scene at the time.
The latter is also supposedly why things changed in English (beyond light and bright rhyming with night). All the stuff about light was stuff that was depicted in nativity paintings. Including the verse you mention, which is actually about the infant Jesus’ laugh:
Silent night, holy night,
Son of God, oh how laughs
Love out of your divine mouth,
Because now the hour of salvation
strikes for us, in the birth of Christ!
My (first) attempt at a poetic translation this time:
Silent night, holy night
God’s son laughs, with delight
Love pours out of his holy face
Telling the hour of redeeming grace
In the birth of Christ
In the birth of Christ.
One that I didn’t question as such, since to me it made perfect sense, is this from “We Wish You a Merry Christmas”:
#Good tidings we bring,
To you and your king#
I was an adult before I realised the intended word is “kin”. But I think either works, just about.
Tedious explanation: at the time the carol was written, most transport was by horse, and harness horses customarily had their tails docked (or “bobbed”), for no reason at all except it was the fashion and they thought it looked good. Their pathetic justification was that the horse’s long tail might get caught in the traces (harness) at the back, alarming it, and it might bolt with the carriage behind it, overturn the carriage, flinging the occupants out onto the road and maybe death. And a good thing too if it had I say. Docking was cruel and painful.
Yeah, I know. But as a kid, “bells on Bob’s tail ring” is what I heard!
The part I didn’t understand about “Good King Wenceslaus” was why Stephen was having a feast outside in the snow.
Couldn’t he have picked a warmer day for a picnic?
OK, “Angels We Have Heard on High”, the stretched-out word in the chorus: As I kid, I thought it was just “Glow”, and didn’t really give any thought at all to the brief “-ria” at the end.
In this morning’s newspaper, The Family Circus has a picture of Dolly singing:
“Oh come, let us ignore Him . . .”
‘Love and joy come to you, and to your wassail, too’
Should kids be hearing this?
“We three kings of Orient are
Bearing gifts, we traverse afar.”
As a kid I was not familar with the word “traverse”. I thought they were singing about how they “travelled s’ far”; kind of a hillbilly-ish pronunciation of “so far”. And was annoyed at how dumb of a lyric that was. 
We sang:
We three kings of Orient are
Trying to smoke a Raleigh cigar
It was loaded, it exploded
BOOM!
We two kings of Orient are
Trying to smoke a Raleigh cigar
It was loaded, it exploded
BOOM!
I one king of Orient are
Trying to smoke a Raleigh cigar
It was loaded, it exploded
BOOM!
God rest ye, merry gentlemen…
Michael O’Donoghue re: “We Three Kings”:
Excuse me? They traversed the fountains? They were on such a tight schedule they couldn’t go around the fountains? I’m sure the locals really appreciated these three yobbos leaving a trail of steaming camel turds straight through their fountains.
What exactly is a “partri-jinapare tree”?