Madamtremain - true, lots of families have a routine where they eat dinner later, usually those are the DCs I see being picked up from school getting a substaintial 'snack' (like a sandwich) to keep them going. however, if they OP's DD doesn't have that routine every other day of the week, not getting to eat until closer to 7 (by the time her Dad has picked her up, got her to his house and cooked), she's going to be starving.
My DCs get fed between 5 - 5:30, but then we do eat as a family, and I know i'm unusual for having a DH who's work hours are shifted by a couple of hours compared to everyone elses so is home by 5 most nights.
Being in bed at 7 is a little early for a 5.5 year old, but not massively so, and if she's a child who needs a lot of sleep/ has to be up early to get to school/breakfast club on time, then not that odd. Eating at 6:30ish would still leave little time for bedtime routine....
OP - can you talk to your exH about this calmly, if he can be there for 6pm, so feeding her closer to 6:30, you might be able to push out her dinner by giving a big snack straight after school, but if he's not going to be able to do that, you'll stick to a routine that suits your DD.
BTW - picking up for 6pm or earlier is not unreasonable time for a working parent to be there for, most paid for childcare ends at 6pm, with pick ups between 5:30pm - 6pm. (Nurseries, after school clubs, childminders around here all expect pick up by 6pm, so before). He is entitled to put in a flexible working request to be there to pick up on time. If you went back to work, he'd have to pick up from the childcare on 'his' night and would need to be there before 6.