I would drop her off for rehersals and go and find something else to do until 5pm, quick looka round sale after and away by 6:50pm. It seems a bit much for one night as presumably some of the Brownies will be manning stalls.
I would try to work something out so she doesn't miss out. I would explain to DD that if the little ones play up you may have to leave the service so they don't ruin it. that way she knows why you had to go out.
Perhaps go and get a snack for the little ones while she is in the rehearsal. Or even just take them for a walk. There must be some shops or a park nearby.