[name_f]Emmeline[/name_f] [name_f]Grace[/name_f], called Millie-Grace, is so sweet!
I do think the full name is a bit long for everyday use and people are likely to drop [name_f]Grace[/name_f]. My recommendation for when she starts school would be to either introduce her as [name_f]Emmeline[/name_f], or introduce her as “Millie-Grace, it’s a double name” and be clear and firm about calling her by both names. Of course, she also might have her own preferences by that time!
FWIW, I like the name you chose better than Lennebelle. 