I have left the Red Fox alone for probably close to 40 year now. Since then I have just observed their behaviors while out hunting other predators or observing active Red Fox den sites in the spring. It is the alpha female who tends to her kits. Along with teaching them various aspects of life. She is the primary teacher of her young.
Red fox kits are weaned around 5 wks old. Typically 5+ weeks old, more so. However, they will still suckle provided their mother tolerates their needle teeth. She will take some of her litter out in the evening hours on short jaunts. As they learn their skill set. As for a straggler in that kit group. She does not wait for them. From what I have witnessed.
The alpha male his duties include bring kills to the entrance hole & dropping it off. As well as defending his kits. IF... the female dies. He will take over her duties. Strange as it may seem. He always drops his kills at the entrance vs any exit hole.
Early in the am. The female may go on short hunts. Leaving her kits back at the den site. When she is gone they hang pretty close to the entrance hole or remain in the den hole. I've observed many females raise their litters. They have taught me plenty & I thoroughly enjoyed my time with them.
When the kits get to around 11-12 weeks old. The female does not pay much attention to them. As they are pretty much on their own by then.
While observing Red Fox near the den site. I have found they will often dismiss deep based sounds I've made to see their reactions. High pitched sounds does alert them & peaks their interest.
Some have spouted neither a Red Fox or a coyote. Does not/will not use an old den. Other than for rearing. I have found that not to be true. They will use an old den. If pursued, injured & blizzards to name a few reasons. So empty is not always empty.
My only interest is wild canine Biology & Behaviors.