I know it is only very early days but DD is upset about leaving all her friends,
I know it is very hard for 10/11 year old girls to understand but in my experience friendships rarely last healthily from primary into secondary school. I am sure there are exceptions but moving onto secondary school is such a great time to re-invent yourself and make friends with those people the 11-year-old relates to rather than those the 5-year-old related to.
Dd did move on with 3 friends from primary - none of whom she now sees at school at all, although they still go to an out-of-school activity together. Similarly with her friends who went to other schools - all of them went with at least 1 or 2 peers, none of them are now close to each other in the school. In fact dd has stayed closer friends with girls who went to different schools. Her friends from her new school are great and seem to have a lot more in common with her.
I would try and sell it to her as an opportunity to discover new people (assuming you can deal with the journey!)