Take a step back.
Look at what you do enjoy in the job (or did, if you can't find anything positive at the moment.) Take particular note of anything you'd like to do more of.
Look at what you really hate, and again, take particular note of anything you'd absolutely hate to do again.
What is it you particularly find stressful? Do you think you'd enjoy your current position if you had more support, and/or more training, or is it just too much at this time of life, with young children? Are there individuals who make it harder (what about the same position, different manager?) What about the organisation type? Public, private, large, small, UK-only, international? Answers to those questions might help you develop a job profile of your ideal job, and even if you can't move immediately (training, experience, time, opportunity,) it can make a difference to have something to focus on and aim at.
Is there anyone else who you could give some of your workload to? Do you have anyone reporting to you, to whom you could delegate (and help develop them)? Or someone in a similar position, who could take on one of your projects?
Does your employer have an employee assistance programme at all? If so, look into whether they offer any sort of counselling, to help you deal with the stress, and also if they offer careers counselling, so you can work through your options. If you're in a union, it's also worth checking what they offer (mine does careers counselling.)