My DD spent about £50 buying her 5 guineas (2 this time last year and the other 3 just before xmas) and has spent in excess of £400 in vets bills already.
The older 2 were having some massive fights and she decided to get the aggressive one castrated (at a cost of nearly £200) when he drew blood, including splitting his brother's lip in 2 places (luckily his brother avoided stitches).
One of the younger 3 was having trouble eating and lost a lot of weight - it turned out his teeth hadn't been meeting properly so had sharp burrs on them, causing blisters and ulcers on his tongue and cheeks so needed an operation to file his teeth down and will most likely have to have this procedure many more times, at £150 a time.
The same lil guy has also broken his leg - the vet was good and said there was no point in a £100 x-ray as the treatment would be no different. He has had to have daily painkillers (Metacam at about £10 for 6 weeks worth, plus a £40 consultation fee). Luckily his leg seems to be healing otherwise he'd need a £200 operation to remove the head of his femur.
This, plus antibiotics for 2 upper respiratory infections, another leg injury, an ulcerated eye and mange 
has made them very expensive little furries!
You should also be aware that they can live for up to 8 years (one of our first ever guineas died at the grand old age of 7 years and 4 months) so are a longer commitment than other small furries.
Don't forget the cost of food, bedding (esp. as wood shavings shouldn't be used) and hay. Just lately DD has said she thinks it'll be cheaper in the long run for her to buy an acre of land to use as a hay meadow than keep buying the hay to meet their insatiable appetites! Even with spending 8hrs on grass during the day when the weather is nice (we purposefully keep the grass in back garden longer so guineas have more to eat), the second they are brought in at night they go straight for the hay and food bowls.
We have had guinea pigs before from when youngest (DD2) was about 7 but couldn't be trusted to help with cleaning out/feeding etc. I'm not sure if I'd trust a 3 year old to not accidentally hurt a guinea pig, but with careful supervision, I'm sure guinea would be fine. You also need to bear in mind that they can be wriggly, fast little critters and can be easily dropped if not careful. I agree with ragged that your DD would enjoy them more when she's older.
They also need a LOT of room to live in - the indoor cages sold in pet shops usually aren't even big enough for 1 guinea pig, let alone 2, and they really should be kept in pairs. It's also better that they are kept indoors where they can interact with the family, but not really in a kitchen because of the temperature fluctuations, or in a child's bedroom as they really should be a family pet IYSWIM.
This website gives guidelines on what should really be the minimum size requirements for guinea pig housing.
Sorry for the super long post, I just wanted to give you as much information as possible.