The toddler has no tombstone because there are no tombstones assigned to newborns and toddlers, nor are they capable of haunting so they go directly to the Netherworld where they can never return.
Also you generally don't get tombstones for any Sims that are not part of the active household unless they are pertinent to the games programming like several EA Sims are or you are present when they die, and again there are only tombstone and haunting ability for child age and up.
The games population controller picks newborns and toddlers specifically because of the lack of ties to the neighborhood as they cannot be co-workers or make friends until they turn child age. Because the controller sees your hood as already over populated babies just create more needs for the game to add to the population for the babies starting with child age and eventually these babies would grow up and have babies of their own. As ghosts are also counted as the general population of the town, any age sim from child age up can add to the town's population even after death, but newborns and toddlers - like I said before cannot.
Elders are usually the last to go, especially elder females because they can only add their ghost which is the same thing as having them still alive actually. A lot of simmers on other sites have cut down on newborn and toddler deaths by reducing the number of ghosts they allow to exist in town by befriending using their tombstones to send the ghosts to the Netherworld and then they are gone for good. Or you can ressurect a ghost back to full life, and when they die the second time they will go direct to the Netherworld as well. They can't die twice.
Or don't add too many sims to you hoods. I usually use just the sims that come with the hood and one couple (Two at the most) I make for myself in CAS, no more. If I want my own sims in the game only I go to CAW and put in a world with no Sims and can have a world with just my Sims. I usually add from 12-20 of my own families to those.