Anecdotal but I grew up in in Hingham, MA that has one of the oldest cemeteries in the country. I used to walk through quite a bit and there was lots of what I am assuming was family plots that would have smaller headstones that would say "baby", and or "son/daughter", etc. Seemed so odd to me. I could only assume these families died at the same time from something tragic. But there was lots of plots that were like that.