Your concern about rodents and ticks is understandable. However, the previous homeowner probably got it right with Pachysandra. Pachysandra is ideal for the situation you describe with the slope, shade and deer. Maybe you could try thinning out the ferns a bit to give you some more comfort from a visual perspective.

This is probably right. Can I cut them back or mow the pachy to make it less comfortable for vermin once I get rid of the ferns?

Sweet woodruff might work as an alternate to the pachy. It's shorter than pachysandra, but not sure if it's short enough to be rodent/tick proof. It likes shade, though not sure about its deer repellant attributes. I personally find SW to be much more visually appealing than pachysandra.

Eliminating established pachysandra and ferns is a huge job. How about making raised walkways or paths through it instead? You could use tree log rounds or even build a wooden walkway that leads to a destination (a treehouse in the oak tree?) -- all for the price/effort of changing the plant material. Plant tons of bulbs, some bleeding heart and hosta, and it will be a magical backyard for your children.