I'm no medical expert and I have never experienced a fox attack. Nor do I feel like I can judge whether she should be euthanased or not. However, my understanding is that a fox bite, regardless of how small will mean that a bacterial infection is likely over the coming days due to the nasty bacteria present in the saliva of a fox. Therefore it is often the case that antibiotics are needed to help the bird get over the infection. If it is a very, very small wound then maybe you could just try topical antibiotics, betadine, saline etc. but it may not be enough.
The other thing to be aware of is that often when a fox attacks it will shake the victim causing internal tissue to tear. So it can be the case that a wound looks minor but there has actually been severe damage inflicted internally. As a wildlife carer I find this is often the case with ringtail possums attacked by cats and dogs. They get shaken and while you can't see much damage, their bodies are coping with a lot of injury internally.
Finally you need to be aware of whether the bird is in pain. It would be cruel to let it linger for days if it is suffering a lot and in pain.
Your pekin will no doubt still be in shock and by keeping it warm, dark and quiet you are doing your best to help it get over that initial trauma. It may be the case that once the initial shock wears off the bird gradually recovers mobility and balance - I mean over a period of days, possibly weeks. If you do make the commitment and the decision to see if the bird will recover then for now I think continuing to keep it warm, dark and quiet is the best way to go in the short term. If a vet visit is an option then that would be an opportunity to get a professional opinion about whether it is a viable situation and also to get some antibiotics if necessary. If not, then just make sure it is eating and drinking and not in too much discomfort and review the situation regularly to decide if you think it is in the hen's best interests to continue the attempt to pull her through the injury and shock.
Also, consider feeding her something like the rickets diet to help give her the strength to heal and recover and you also need to make sure you tend to the wounds; bathe them to remove any dirt, and apply something like betadine.
These are just things to consider while you wait for a medical moderator to give you a more experienced opinion. I really hope she is Ok and I understand it must be a horrible thing for you and your family. I would be devastated and very conflicted about the best course of action. Good luck.
Chicken07 has just posted and raised similar concerns about the likelihood of infection.
Mother to a happy bantam flock: Beatrice and Valentina (Rhodies), Aphrodite, Violet and Blue Ivy (Australorps), Minnie (Silver-pencilled Wyandotte) and the buff Pekin princesses, Penny Lane and Pigwidgeon.