God, you sound wonderful (step-in)parents...bless you
Not sure how you can deal with this practically (don't live in UK).
The only thing I can say is to tell her in an age-appropriate way what is going on.
She needs people who are honest in her life.
For what it's worth, I have a friend who has split up with her child's father, while both are pretending it's not so ("daddy has another place to stay, he's away on business")...I can see lots of future problems, after all if the people who take care of you can't be honest, who can you trust?!
I can understand their ("protecting her") logic, just don't think it is right....
She's coming up for 7 by the way...(been going on for about 2 or 3 years).
I just feel sorry for her when she finds out....
Wishing you all the best, you are doing a fantastic job.....