There is a statement in the Photo Privacy page of the Help file saying "Please note that we never recommend uploading sensitive images of young children and that there are restrictions about the content of images in the Terms of Service." I understand that you're not proposing to upload images of the children themselves, so my direct answer is that I have never seen any rule or policy statement here that specifically prohibits it. If you are posting the photo on the profiles of the living children, which are presumably Unlisted, or the profiles of one or both deceased parents, and those profile(s) are still Private, then the photo would be private as well. Members of the trusted list could see it and others could see a thumbnail version. If you need the photo as a source, and it is on another web site, such as FAG, you could sidestep the issue by just linking to the other web site.
Having said all that, Grandma had a rule to live by: "When in doubt, don't." If the photo isn't really a critical source, and you can dance around the factual data without actually posting it, you might consider holding off. You can always decide to post it later if the rules liberalize a bit, or if the children are old enough to say they don't care. (My $0.02)