What would you do?

My daughter turns 5 in may. She always comes home and tells me things kids say to.her. they are mean things. She had picture day today at school,she looked beautiful. Well some kid told her that her "dress wasnt pretty". This had to of made her feel terrible because she came home and was reslly angry before bed and said to me "youre not very pretty". Which only hurt me because i knew that's how badly it made her feel because she was mad when she said it. My daughter has only ever been showered in love. She doesnt know bad things to say or not nice things to say. I told her all that mattered was that " she loved her dress, and that she thought she was beautifulin it" but she is hurt everyday by these kids words. It seems this age is finicky. One day thsi girl will say shes her best friend and the next day she will say she isnt. And then my daughter is sad about it. What do i say to her? I dont rememener children being this mean when i went to schoool.