Anyone who enters a child into such a contest would face up to two years in prison and €30,000 (£25,000) in fines. The legislation must go to the lower house of parliament for another vote.
The Senate approved the measure 197-146, as part of a law on women's rights.
The amendment says it is aimed at protecting children from danger and being prematurely forced into roles of seduction that harm their development.
Beauty pageants involving girls of all ages heavily made up and dressed up, regularly spark public debate in France.
Conservative MP Chantal Jouanno, who put forward the amendment, said: "The foundations of equal rights are threatened by the hyper-sexualisation that touches children between six and 12 years old."
Michel Le Parmentier, organiser of "mini-Miss" pageants in France since 1989, said he was disappointed the draft law involved an overall ban.