I don't know if it's normal for "normals" but maybe it is for a parent with MI. I remember when my kids were small I'd create scenarios in my head about what I would do if a disaster happened. One I never figured out was if our apartment caught on fire. I had 3 kids at that time. I could carry 2 out at one time, but not 3, so which 2 would I grab first? Luckily it never happened. And when I had my last child I had post-partum depression and I was worried more than necessary that something would happen to him. Nothing bad happened, he just turned 21.
I think especially when a closely related child has died it will certainly crank up your motherly anxiety! I would call that perfectly normal. You know you can't protect them at all times and things will happen beyond your control.
Are you getting treatment for anxiety? Not that this kind is bad, but it can get out of hand. Don't let worry consume you. You should be enjoying the time you have with your kids instead of worrying about what might be.
Be sure to tell your doctor about your anxiety and concerns.