I soaked the puppy complete I was feeding my bitch with Lactol made up to bitch strength, also gave raw tripe, beef, cooked fish/eggs, and grated cheese - not all at the same time ;) 4 or 5 smaller meals a day. She also had Lactol to drink as well as water being available at all times - she rarely touched the water, but would drink the milk no problem.
Always took the meals to the bitch in her den as she wouldn't leave the pups ;)
If the bitch really isn't producing enough milk and the pups aren't gaining over a few days, fennel seed tea may help to increase milk production, it helped with a bitch of mine and I've used it myself too ;)
Why would the vet think she needs weetabix?

she needs
more calories and protein in a smaller package so that she can make milk, weetabix is the opposite - few calories but bulky