Hi!
Babies can sometimes have weak back legs from being very cramped in the last days in the womb. However, this should right itself within a day or so. Try to very, very gently massage the legs and see whether that helps.
If there is no improvement, then please have the baby vet checked; they can also be born with problems. You can reclaim any vet arising from the pregnancy from the pet shop as part of your customer rights for having been sold in effect faulty ware. You can also ask for a second cage for free in case you have a boar pups.
New guinea pigs: Sexing, vet checks&customer rights, URI, ringworm and parasites
Please read up on mother and baby care; especially our diet advice in order to protect your mother's own bodily reserves as she is still a growing youngster herself.
You can find our information neatly laid out via this guide here:
Pregnancy, Mother & Baby Care Guides
Additionally you may find our new owners information collection a very helpful resource that you want to bookmark and read/re-read at need. It is one of the most comprehensive currently around; we are addressing all the issues that we get the most often asked questions and concerns in practical detail that does not gloss over the sticky bits:
Getting Started - New Owners' Most Helpful Guides