You can write your own will. There are self-help books and forms that you can buy.
HOWEVER, if there's anything even a little complicated about your situation, I'd suggest at least having it reviewed by an attorney to save your heirs all sorts of problems.
If the two of you are not the parents of the children (if one of you is a stepparent, for example), then you really need an attorney since the other parent would have first rights in most cases.