Samantha Cameron braved the cold in a Christmas jumper and purple heels to greet young fundraisers outside 10 Downing Street.
Prime Minister David Cameron's wife congratulated the schoolchildren for raising more than £500 by taking part in Save the Children's Christmas Jumper Day, where people wear a festive sweater and donate £1.
Mrs Cameron, ambassador to the charity, told the children from St Nicholas Preparatory School in Kensington that they looked amazing in their homemade Christmas jumpers and said she was impressed.
Speaking about the day itself, she said: "It is a fun and simple way to make a real difference to the lives of vulnerable children across the world. It is heart-breaking that eight million children under five die each year and I'm proud to support Save the Children's vital work."
Mrs Cameron's woolly red jumper was bought from the charity's shop and the money donated will help feed children in the world's poorest countries.
This is the first time the charity has used Christmas jumpers to raise money, and 650,000 people across the UK are expected to take part.
Justin Forsyth, Chief Executive of Save the Children, said: "We wanted to do something fun that could get everyone up and down the country involved so this seemed perfect. It also has a serious purpose which is that we want everyone to raise money by donning a jumper and donating a pound."
The pupils from St Nicholas school decided to raise money for the charity after hearing about its work.
Armel Esper, 10, said: "We chose the charity because if the children died early they would not have lived for a long time and we are children so we wanted to save their lives."
Her classmate Nathan Leon, 10, spoke about meeting Mrs Cameron and said: "I was quite nervous at first but it went surprisingly well. She was pleased that we had raised the money for the charity." Teacher Kate Keightley added: "She must have been cold in her heels. She needs Christmas socks."