Redundancies are horrible at any time and stressful for those that stay and go. Whilst length of service, performance, and track record are important so is the ongoing success of the business. Which is after all why there have to be redundancies.
It is critical that the staff who remain are those best able to serve the current business need. The people who've been there longest may not be the one who has the most relevant needs and skills to fulfill the requirements of the current clients/ contracts.
In my last company we had a list of 10 assessment criteria and marked each person out of 10 for each so total potential score of 100. These included all the usuals + a few others it was something like:- length of service, sickness, performance, disciplinary, skills bank, business knowledge co. specific, projects currently involved in, fit within the workplace.
Each person was assessed by their line manager and then a weighting was given to each line managers scores so that on average individuals were equatable. We wrote a reference for each employee on the back of the assessment form. The reference was positive so one persons might say John Blogs is a good team player who is punctual and has a low sickness record.
John Smith is a good team leader and player who is punctual and has a low sickness record.
Generally a few instances of good behaviour were also highlighted for each employee. When the redundancies were then conducted employees whether staying or going got a positive performance review and had a ready reference for other jobs. I contacted all the local jobs agencies and produced lists of the contact names, numbers and jobs in the right skill sets. Which were passed to those leaving. Also gave those who were leaving the opportunity for early paid get out subject to handing work over, to minimise disruption on those staying.
Not a nice situation you will get through it. Good luck.