I've said and done some quite embarrassing and amusing things, especially when I was younger.
When I was 5 years old, me and my mother were at her friend's house. There was also a Police Officer there (I think he was a friend or relative of my mum's friend), and seeing me sitting on my own he decided to talk to me. He asked me how my mother drives and my reply was "My mummy drives really really slowly, round and round in circles and she gets lost a lot."
A few years ago my mum's best friend phoned to ask how she was, and I answered the phone (I think I was about 11 or 12). My mum was on the toilet, and I've never been very tactful, so I told my mum's friend in my most polite voice "I'm afraid my mother cannot come to the phone right now, she's having a poo."
When I first started Secondary School (I was 11) I was in the same PE changing room as one of the so-called "hard girls" who was in a gang and apparantly quite tough. She decided to pick on me by asking me why I was wearing such large panties, and my reply was "Because I have a big bum, DUH!" Apparantly a lot of people were expecting her to try and beat me up at some point after that, but she never did.
This next one is more a case of me doing something embarrassing/funny as opposed to saying something funny.
I used to go to a holiday playscheme because my mum had to go to work and I couldn't be left alone in the house. I was about 9 years old when this happened. I can't quite remember the incident, so I'm going to quote what my friend said on MSN when she explaining the incident to somebody else.
"Ok so I went to playscheme with Squirsh when I was younger. In the dining hall somone pointed out a dead rat. All the children ran off, all but me and Squish. Squirsh stood there flapping her hands, asking if she could put the rat on her head and then started dancing around and trying to touch it, and she had to be restrained...I just sat there XD"