You'll want to do what Imbrium said about the food. Pick the treat pieces out(a pain I know) and just feed the plain green pellets. Then get a new food and over 2 weeks gradually switch over to the new food starting with a very small amount of the new food and mixing it with the old food. Mske sure you get the exact same kind as they are feeding her right now, when you bring her home. You don't want to change foods suddenly. The reason you don't want to switch a rabbits food suddenly, and why you very gradually and slowly transition onto a new food, is because rabbits have a very sensitive digestive system. If you change foods suddenly, it upsets the balance of bacteria in the digestion, and as a result can make the rabbit very sick, even to the point of dying. So by transitioning to a new food slowly, you are giving the rabbits digestion time to gradually get used to the new food. Usally the best way to know it your rabbit is having digestive problems is if their poop changes. If the poop gets smaller than normal, is irregular shaped, or is soft and mushy, then it is having a problem and you need to adjust it's diet. If a rabbit ever stops pooping, then it is an emergency.
A lot of us use the wood pellets in our litter boxes cause they're cheap and work well, but there are other kinds that are ok for rabbits too. Yesterday's News pellets, Carefresh bedding, aspen bedding, Kaytee soft granule bedding are all safe for rabbits. Don't use cat litters as they cause respiratory problems with rabbits and the clumping ones can kill rabbits, also the cedar and pine shavings have oils in them that aren't good for rabbits either. Sometimes with picking out the right litter and litter box will depend on how easily your rabbit will train. I had one bunny that wouldn't use her litter box til I changed the kind of litter I was using. I have another rabbit that had a corner box but she was always peeing on her floor and digging the litter out making a mess. I got a larger rectangle kitty litter box, and that was the end of the messes. It also worked well because on one end of the box she peed and on the other end I would put her hay so she could poop and eat in the box.
You probably never thought that getting a rabbit could be so tricky, but once you learn the good and bad things for rabbits, then it gets easier. Just remember that most rabbits are big chewers, so you need to make sure everything in their environment is safe to chew on.
If that petshop is saying that that 'treat food' is ok, but feeding adult rabbits, young rabbit food will kill them, then those people don't know anything. I've found that most petstore people don't really have a clue about what is really good for rabbits, so you're pretty much stuck doing your own research and better off not listening to their advice. Typically young rabbits are fed an alfalfa based pellet because it is higher in protein and calcium, and that's what young rabbits need. Some people will feed unlimited pellets til the baby is about 6 months old, and some people will feed limited pellets to young rabbits, but in a higher amount than an adult rabbit would get. For my baby rabbits, they would get a poopy bum on unlimited pellets so I would limit my baby buns pellets and free feed timothy hay. When a rabbit is around 6 months old, alot of people wil switch their rabbits to a limited timothy pellet because it has less protein and calcium, but some people will also feed an adult rabbit an alfalfa based pellet in limited amounts, and it doesn't kill them. If they meant feeding that treat food would kill an adult rabbit, yeah it could because of all the carbs and sugars in it, but it can also kill a young rabbit as well. Rabbits are very sensitive to too many carbs and sugars, some more than others. All you need is a plain alfalfa based pellet without corn in the ingredient list. I like sherwood forest food, but I've also just gotten a plain alfalfa pellet at walmart as well. And you always want to feed unlimited grass hay to your bunny, even when she's an adult as it is good for their digestion and helps prevent digestive problems. I get timothy in a bale for my rabbits. If you have a horse feed store with it then that is by far the cheapest way to go.
I'm glad you are researching what will be best for your daughter's new bunny. Unfortunately most new rabbits don't live very long because the owners don't do any research and they make a lot of common mistakes, that end up being bad for the rabbit. The library section on this forum has a lot of helpful info as well as the HRS website. Here's the link, and I hope your daughter loves her new bunny and has a great Christmas
http://www.rabbit.org/care/veggies.html