The charity Missing People will tweet a different appeal for a missing child every half hour for 24 hours, using the #FindEveryChild and #TeamBigTweet hashtags.
The celebrity support, in the form of tweets and retweets, follows on from JK Rowling and Simon Cowell last year.
The Home Office and other large-scale companies including Royal Mail and Hootsuite have also joined the campaign by retweeting messages.
"Our aim is to bring missing children to safety and end the heartache for families desperate for news of their missing loved one," a spokesperson for Missing People said.
Now in its fifth year, the charity revealed that since last year's Big Tweet, 19 of the children featured in appeals have been found.
The charity is urging others to take part by following @missingpeople and retweeting as many appeals as you can.