Congratulations on your imminent arrivals!! Guinea pigs are fantastic pets! It's great that you are buying two together as they are much happier in a "herd" than alone.
Be prepared that two boys together may get along fine - but they may not. It depends entirly on the individual gp. If they are content together in the cage as babies then that's a very good start but look out for bullying/over-dominant behaviour (particularly when older).
First of all, I would check that they really are both boys by gently looking at the genital area (open very gently). If one of them seems to have a 'Y' formation then it is female and you'll need to separate or swap them, or babies will follow shortly.
For the first six weeks, they will be very, very nervous so you have to give them time to settle in. (This may be hard for your dd, depending on how impatient she is to get her hands on her new pets!! - but it's worthwhile going slow and building trust so they settle in gentle and gradually build up their trust in you.) Just quietly and slowly feed them a fresh treat twice a day at the beginning and then , when they accept that, quietly take them on your lap and stroke gently - no longer than 5 mins at a time. MAke sure you interact with them every day but also allow them space to retreat in peace and quiet too.
Ours are very friendly now and will sit and even sleep on our laps quite comfortably. They squeak when they need to pee, or sometimes just toss their noses up quite forcefully - so you know to put them back in their cage. At the beginning though, like any young animal, they will pee on the floor until they are trained. Most gps will poo on the floor every so often ...
GPs bite only very, very rarely - and only if they are handled too forcefully. Always make sure they are supported underneath when you pick them up and keep them close to your chest. Also, they dislike having their rear ends stroked but like being tickled behind the ears!
We feed twice a day - handful of hard food in the morning and a plate of fresh food in the evening. Carrots, endive, cucumber, broccoli and fresh sweetcorn are their favourites. Go easy on parsley and don't feed rhurbarb or too much dark cabbage because of potential build up of oxalic (sp?) acid. You probably already know that, like humans, they don't manufacture their own vit c, so need fresh food every day.
Gps love routines - ours even have a "set" bedtime at 11.30pm. They have great individual characters and can definitely make their feelings known to you if they want anything!!!
They have the same sensitivity to temperature as humans but they can't sweat so in the summer, they always need access to shade and water. In the winter, we keep ours indoors in our basement (garage fine too). It is possible to keep them out in winter but they do not react well to damp or drafts and so you need to make sure in very, very cold weather that you have protected the cage with old blankets and waterproof coverings etc (at the same time it's important that the cage can "breathe"). Tbh, I wouldn't have kept them out in the bitterly cold snap we had over Christmas. In the summer, we let ours run out all day in our (walled) garden and bring them in at night (because of cats).
Oh yes - you need to clip their nails every 3 months or so - and occasionally they enjoy a nice tepid bath!
Can't think of anything else except to say you will love them.
Good book recommendations:
here
here
Don't waste your money on RSPCA guide. It is too basic and inaccurate in parts.
Good luck!