We did an 'awful egyptians' party for dd2's 6th birthday, it went down well. We did some craft making - decoupage sarcophagi and pass the parcel with some egyptian multiple choice questions and some big hints. Don't eat cleopatra smartie went down well, same as don't eat tommie smartie with a name change. What's the time mr crocoldile worked well too.
My 7 yr old had a Victorian themed party (odd choice but the class had bn doing Victorians at school) so they all knew about it. Have her friends been doing anything like that? Or, why not broaden it out? Say children can come dressed up as 'anyone in the past'. So it becomes a History Party. They may have been doing Romans at school.
You could then just have historical takes on traditional party games (we had pin the seat on the penny farthing) and pass-the-parcel could be easily themed with a traditional toy inside. The children also made peg dolls and a nosegays. The Victorian style high tea went down a treat - scones, sweets, cake, lol. There must be Georgian party games (I don t know much about Georgian times but I found lots of info through a google search for the Victorian party). Of course, you need to dress up as well! What fun.
I'd be tempted to get your DD her special outfit and then beg/hire/buy/borrow some fancy clothes and have a big dressing up box for the visitors.
Maybe have a craft activity where they make their own wigs out of paper somehow? Or use face paints and stickers to give themselves Georgian make-up (white base, lots of red cheeks and stick-on eyebrows and beauty patches). Or forget about the Georgians and have a Rattus Rattus treasure hunt round the garden.
You could make some of the food a bit Horrible (worms in jelly?) - but I wouldn't go overboard with that as 5yos can be a bit picky at the best of times.