Unfortunately, even if you did successfully remove the picture from the original website, there are sites such as Wayback Machine which essentially serve as web page museums. I was able to access a version of my company's website from 2001 in five seconds.
And unfortunately, an image is just one of many ways kids can find out about the past. I know somebody who found out through a flippant remark offline that her mother committed suicide (as opposed to an actual accident). The death had happened decades ago and it still tore her heart out.
You will soon need to figure out a response when your son's children find out. It is infinitely better that they find out from you rather than a website or a stranger. You can't erase the past, but you can help your son's children work through it.
I'm so sorry for your loss.