Joshikousei To Ouji Chan

Ume Daidouji is determined to make lots of new friends after starting high school in a new town, but her first day at school doesn't go so well. After school she starts a new part time job at an ice-cream shop. While distributing flyers and worrying about her new life she is hit on by some sleazy men.

The night in shining armour who saves Ume is a young primary school aged girl called Momo.

This is the beginning of their friendship, at least until Momo confesses her love for Ume...

(Source: MangaUpdates)
This entry includes the two bonus chapters (5.5 and 10.5).