It's not about "chest out". Her chest is sufficiently out and yet there is still a roach in her back.
The key to good equitation is a strong core. If your daughter is keen on riding and riding well I would suggest to you and her to start getting her into some form of core strengthening or simply cross training exercise. Whether this is a structured yoga, pilates or gymnastics class or something less structured like playing pick up soccer with friends or going on bike rides together a few times a week, all of it will help to improve her core strength for riding. Many of the top training barns in the US have a gym or studio and have a personal trainer come in to train the riders a few times a week. It's not about fat it's about fit! Being too skinny is also not desirable for a rider.
Anyways! Onto the critique. With increase trunk and core strength your daughter will be more effectively able to push her midsection forward. Think about your position leading with the belly button in front of everything. Here is an exaggerated picture of the belly button leading: http://2.bp.blogspot.com/-FSiCjmGFjO..._4696small.jpg
The horse is in extended trot, so the rider is really having to engage in the core and the trunk to stay with the horse, see the chest is out, but the belly button leads with an engaged core to stay with the horse. If we could see the rider from a complete side view, and without the jacket, her back will be flat with her shoulders flat on her back and her pelvis in a "neutral position" with a natural curve to the lower back - the spine always remains in "neutral" with a flat back: http://t3.gstatic.com/images?q=tbn:A...kc8O1SfxrREDsD
Once she is riding with a leading belly button and an engaged core, the pumping should cease. She needs to think about keeping her shoulders completely level and still and let her seat absorb the motion of the canter. As well, to keep her hands positioned correctly where they are in the video, she will need to focus on pushing her shoulders and elbows down. This requires much core strength. The final thing - and this should also be corrected with an engaged core and when she stops pumping, is to stop sticking her neck forward. The neck is an extension of the spine and again, needs to be in a neutral position. This means engaging the muscles at the back of the neck to tuck the chin and elongate the back of the neck. In yoga this is called "humble chin".
Once the core is engaged and the whole spine aligned and neutral, she will have a good foundation for good equitation. Until the body is stacked and aligned, all other position things (like heels down, thumbs up, etc..) are moot. Until she is riding in a correct balance it will not matter how those aids are given to the horse as he will always be following her weight and alignment. He is a very obedient horse, but with a balanced rider can be exceptional.
Overall I think for your daughter's age she is doing very well, however needs to focus on the fundamentals to form the basis of good equitation for life now. I can tell you from experience that re-learning over top of bad habits years down the road is extremely difficult!
Good luck and keep riding :)