The goal here is to find out why Daisy is thin. Is she having constant diarrhea? Not eating? Or on poor low quality puppy food? You should start by getting her on a high quality large breed puppy food such as Hill's Science Diet Large breed puppy
http://www.hillspet.com/en/us/products/sd-canine-puppy-large-breed-dry OR Royal Canin large breed puppy food. Merrick is also a decent quality dog food as well. Then you want to get Daisy's stool tested by your vet for parasites. A prophylactic deworming can also be done (i.e. Pyrantel palmoate or Fenbendazole). And finally treat for any diarrhea or loose stool with added fiber (i.e. Canned pumpkin- not the filling, 1-2 tsp daily) until the stools are normal consistency.
And finally you want to protect Daisy from known viruses and pathogens by getting her started on her puppy vaccinations. Most puppies start their puppy series between 6-8 weeks of age, so Daisy is definitely old enough to start these now. I hope this helps!