Recently acquired piggies here too. We have a sibling boar pair.
They were for DD 4.5 and DS 2.5.
Obv that means I look after them.
They are not hard work and very sweet. Easy to clean out and feed.
Grooming is easy too as long as you get short hair ones. The fluffy ones look cute but involves brushing a lot I think.
I've had many pets as a child. Rabbits, cats, dogs, horse/pony.
Piggies are by far the easiest. Other than a cat actually. I suppose a very easy pet would be a cat.
The piggies take time to get to know you so you need to prepare your DD for the fact that they won't come running to her for cuddles on day 1.
I have a plastic (think its a ferplast one) which is 180 x 60cm. It's essentially rabbit sized but I saw the recommendation to get a large a cage as poss.
I would def recommend piggies as a pet for a child your DDs age. Rabbits can be hard to hold and need lots and lots of attention and cuddles or they end up very sad actually. One of the rabbits I had was a biter with a temper. Quite hard work and I struggled a lot with it (I was 7).
Good luck with choosing your piggies.
Oh and someone mentioned sawdust.
I was recommended something called Megazorb. It's a special absorbent wood pulp which has been heated and vacuumed to extract any dust from it.
It's used for small animals such as piggies and bigger ones like horses (horses don't tolerate dust that well).
The Megazorb stuff is amazing. So much softer than wood shavings and it works out cheaper. £8 for 85l and its extremely absorbent so you don't need wast amounts and you can easily clear bits out every day. All the guinea pig rescues recommended it.