HoHum - I'm Hindu so might be able to help although I just typed a massive post and then lost it so maybe not!
Sounds like your Aunt is having a puja (ritual ceremony) to bless her new house. I've never had a puja myself but have been to many. Just as a disclaimer, there are many different traditions within Hinduism depending on the region you come from. I'll give you my experience and having been to pujas held by others, I find the structure broadly the same...
When you get to the puja, you'll find the pundit (priest) next to an area specially laid out on the floor with the murtis (statues and pictures) of the various deities in front of him and the family concerned sitting next to him. There'll be a ceremonial fire which will form part of the ceremony. He'll chant mantras (prayers) and perhaps sing a few bhajans (religious songs) and if you have a good one, he'll explain what he's doing as he goes along. Most of the pujas I've been to have been very long so eat before you go - it's traditional to keep the day of the ceremony "clean" i.e. no meat or even fast although I wouldn't have thought that's expected of you.
At the pujas I've been to, we've been expected to sit on the floor so wear comfortable clothes and be sure not to point your feet towards the ritual area as it's considered disrespectful. Children are welcome at pujas despite their obvious distraction from the ceremony but I still find it necessary to take mine out at various points (they're 7, 3 and 1) especially in the noisy parts for instance where there's banging of a large gong or blowing on a conch shell. I also find the smoke from the fire sometimes gets to the kids a bit so it's a good idea to park yourself somewhere near a doorway so you can make a quick getaway if needed.
At the end of the puja, your Aunt and Uncle will probably go around the house with a plate of various items blessed by the pundit with a deya (lamp) in the middle to bless the rooms. At this point, someone may come round with a plate with a deya in the middle where people put their change and then take blessings from the deya by placing their hands (carefully!) over the flame and then passing it over their heads. Someone may also come around with sweetened milk - be sure to use your right hand on top of your left hand to take the milk as the left hand is considered dirty. Sometimes there's perfume and chandan marks to the head so your daughter may find that interesting too. There's always lots of different food afterwards - it may be they've catered for the English palate but I always take a few snacks in my bag for afterwards (my kids are fussy and don't like spicy food!) and surreptitiously give them to the kids when no one's looking.
So far as preparing your daughter, I've taken the liberty of googling "puja, children" for you and come up with this BBC video which looks interesting www.bbc.co.uk/learningzone/clips/puja/4799.html
If you live near an Asian area, you might want to visit a dress shop where you could find a traditional outfit relatively cheap - shalwar kameez (long top with trousers and matching scarf) is ideal at that age, I'd say. Oh and before I forget, most hindu homes have a shoes off policy (much to MN's chagrin I'm sure) so don't take Jimmy Choos or anything!
Not sure what else would help but if you have any questions, I'd be happy to answer as best I can or bump for the more knowledgeable Hindus. Hope you have fun anyway - I love pujas!