We used to say "pouce!"when I was little and we had to put both thumbs up.I don't know if the children in France still use this expression.Google translates this word as thumb.Wikipedia isn't my favorite place for researching things, but I did think it was interesting that the article on truce terms refers to the possible use of the thumb in this context as far back as the time of James I of Scotland (early fifteenth century):
The use of barlay as a truce term appears in the 14th century poem Sir Gawayne and the Grene Knight and Tobias Smollett's The Reprisal. It is recorded in lexicographer John Jamieson's 1808 Etymological Dictionary of the Scottish Language as a term specifically used by children to demand truce. A probable variation also appears in the 1568 manuscript Chrysts-Kirk of the Grene, sometimes attributed to James I of Scotland, as follows;
Thocht he was wicht, he was nocht wyss,
With sic Jangleurs to jummill;
For frae his Thoume they dang a Sklyss,
Quhyle he cry'd Barlafummill.
The "Thoume" (thumb) that is "sklyss" (sliced) in the quote above may refer to the thumb having been raised by the man calling barlafummill, a common accompanying gesture to the use of a truce term in Scotland.