I’ll definitely second the suggestions of Gon and Yotsuba&!. Both are fantastic and good reads at any age.
At 11, your neice might be old enough for Sailor Moon, which has been amazingly popular with girls for over 20 years now.
Nichijou is also aimed more at teenagers but doesn’t have any objectionable material. It’s about a trio of school girls for whom everything is serious business.
While his art style is quite dated these days (he was strongly influenced by Fleischer Brothers and early Disney animation), Osamu Tezuka created a massive library of manga from all genres and for all ages and his storytelling skills are phenomanal - Astro Boy is probably his biggest hit and should be available in English.