Babies keep and toddlers keep disappering

@bellybunny  This is a known issue with the game, unfortunately.  It doesn't always happen, but once babies and toddlers start getting culled, it can be difficult if not impossible to reverse the process.  The easiest way to put a stop to it is with NRaas StoryProgression: the mod won't let sims be killed off for no reason.

SP is definitely a heavy mod, but you can mitigate its impact by slowing down the overall pace and the speed of each "manager": increasing the cycle length means SP is doing less at any given time.  You can also limit how much inactive sims are allowed to do, for example preventing them from divorcing or changing jobs.  If you'd like to start using SP and don't know how to set it up, the NRaas site is the place to ask.  There are an overwhelming number of settings, but you can really ignore most of them if it makes things easier.

If you don't want to use SP, NRaas Overwatch will at least help recover some of the toddlers, depending on how they went missing.  But it's not a perfect solution.  The only other viable approach that I know of is to turn off EA story progression in the game settings and manage the town's population yourself, either playing the households while they reproduce and waiting for the kids to age up, or using MC to create pregnancies or instant babies and then aging them up right away.