Remember that they are overwhelmed and fragile now too. I'm sure you've told them that your crying has nothing to do with anything they did, but they still might feel badly that they couldn't make you feel better or at least make you not cry.
If you have not done so, you and your husband need to sit down with your children, individually, and tell them what is going on, in a manner appropriate to each child's age and personality. You also need to reassure them of your continuing love and support and try to keep up the daily routines of the family as best you can.
Even though you say that you don't want this divorce, you and your husband really need to make an effort to be civil to one another and not to argue with each other during this time -- your children deserve that. Also, please be careful not to enlist your kids as your caretakers, especially your older children. You need to care for them.
I would never ask you to promise not to cry in front of your children. You should not be expected to put on a happy face and pretend that you are not sad. But, let yourself cry with your friends and other family members. Tell them what you need from them. Allowing yourself to cry around them may reduce the incidences of breaking down in front of your children. If you find yourself unable to stop yourself from frequently crying in front of your children, I would strongly recommend some supportive counseling.
Since your children know that you are sad and have seen you cry about this divorce, you may simply want to let them know that, although you might cry sometimes, you are always going to be there for them and that you will all make it through this tough time together. It is necessary that your husband convey the same message to them with you. Again, please reach out to others to help ease your pain. Your kids need some extra loving as well from those close to them. You may also seek professional counseling at this time for yourself and your children.