Q: Should you tell your mother something if it is important when she is talking to company? I am 6.

A: Yes, you should (after saying “Excuse me”). Here are some of the things that are important to tell your mother, even though she is talking to company:
“Mommy, the kitchen is full of smoke.”
“Daddy’s calling from Tokyo.”
“Jennifer fell out of her crib and I can’t put her back.”
“There’s a policeman at the door and he says he wants to talk to you.”
“I was just reaching for my ball, and the goldfish bowl fell over.”
 

Now, here are some things that are not important, so they can wait until your mother’s company has gone home:
“Mommy, I’m tired of playing blocks. What shall I do now?”
“The ice-cream truck is coming down the street.”
“Can I give Jennifer the rest of my applesauce?”
“I can’t find my crayons.”

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1981-03-08)
    (Source)

A slightly different version was given in Miss Manners’ Guide to Excruciatingly Correct Behavior, Part 3 "Basic Civilization," "Concerning Children" (1983):

DEAR MISS MANNERS:
Should you tell your mother something if it is important when she is talking to company? I am six.
 
GENTLE READER:
Yes, you should (after saying "Excuse me"). Here are some of the things that are important to tell your mother, even though she is talking to company:
"Mommy, the kitchen is full of smoke."
"Daddy's calling from Tokyo."
"Kristen fell out of her crib and I can't put her back."
"There's a policeman at the door and he says he wants to talk to you."
"I was just reaching for my ball, and the goldfish bowl fell over."
 
Now, here are some things that are not important, so they can wait until your mother's company has gone home:
"Mommy, I'm tired of playing blocks. What do I do now?"
"The ice-cream truck is coming down the street."
"Can I give Kristen the rest of my applesauce?"
"I can't find my crayons."
"When are we going to have lunch? I'm hungry.”