My dd is currently in yr 4 and,along with 5 others, have been taught with yrs 5&6 this current academic year.
Her school is a small rural school,and this needed to happen as her 'real' year group (combined with yr3) was too large.
I had my misgivings as I felt it would,well,rob her of some of her childhood.
She does know about things that she may not have come across until later (violent Xbox games,sexual orientation for example) but she took it in her stride,we talked about it and it's fine.
The other thing is friendships within the 'real' yr4 class have dwindled, but this has been compensated by friendships with yr5&6 girls.
Her confidence levels have also improved hugely.
Afaik,your dd,OP, would have to wait until she was 11 until she would get to secondary school. You would have to talk to your head about how this would work.
Not really much help I'm afraid,and a different situation,but just to say it has worked out ok for us.
Good Luck 