You need to get specific help for this problem. Have you been to CAB? It is free, and they ought to be able to point you in the right direction. Do you have any friends or family who can back up what you say? I am not familiar with the Family Court system, but if you are making a formal application for separation, then official arrangements must be made for the children. If they really are at risk and you can prove it then Social Services should be taking this seriously. Which of you instigated the splitting-up? How are you both planning to live when it happens? Do you own a home which will have to be sold? If you want to have custody of the children, how will you look after them during the day if you are working? You might find yourself in the position of having to move out of the family home, pay for somewhere else to live, and still pay for the upkeep of your wife and children. You really do need to be very up-to-scratch on the implications of what you are up against.
These are all questions which you must get very clear answers to. Ideally you should have a Solicitor to represent you, but they can be expensive. There are two organisations where you might get some help. They are:
www.FathersMatter.com
and
Families Need Fathers - www.fnf.org.uk
Try both of these to get help to sort out your situation before it's too late.
I wish you luck and hope you can come to an amicable arrangement with your wife - you certainly don't want to be in the position of losing contact with your children if your wife fights hard and dirty, but even more important is your children's safety whilst they are growing up.