The death of a child is tragic, not matter the circumstances. Is it reasonable that your husband and his ex should comfort each other in this circumstances? I think the answer depends on how you feel about them. If you completely trust your husband and believes that he is over his ex, then you have nothing to fear. If on the other hand, she still wants him or he still may have feelings for her then I can understand your concern.
If it is a trust issue, then be involved with them. Don't allow him to go to her place without you. Even if you have to pretend to be nice to her, do so because you want to support your man but at the same time, you do not want any hanky panky behind your back. Tragedy has a way of bringing people close or dividing them. The emotions can run real high and become very confusing; mistaking mutual sympathy for old feelings of lust.
I think however, that the child's death should be the focus. So offer sympathy and support. You do not have to be friends with the ex, just standing with your man.
If your husband knows how you feel about the situation and he cares about your feelings, he will use discretion.
There should be no 'running to' but rather offering support and getting through the circumstances and understanding that both have shared past.