Considering the child does not appear to have been actually harmed, and it does not appear to be something that was done regularly, I think much more than a month or so would begin to become excessive. They're young parents, and seem to be genetically idiotic, so I think that this one-off instance is worth punishing for, but not taking the child. The foster system exposes a child to far worse things than tape. If the mother has learned her lesson (and I don't feel we have enough information to judge her either way), then prolonged jail time and taking the child permanently are excessive.

Now if something like this happens AGAIN, I say throw the book at her.