Wow.

Not sure if Anchor Worms are a threat to garter snakes, not sure they can survive the digestive process.
There is some debate if freezing in standard freezers has a low enough temp to kill the eggs or larvae of some parasites. Length of time in the freezer also has been debated.
Why do you think you need to treat the babies?
Do you have an on-call Vet. that you can ask some of these questions or at least get a fecal float done on the mom. Then you will know how to proceed with meds.

What State are you in and have you identified the snake you are working with?