I had a bug in my first ever Twinbrook game where all the nooboos died before reaching school age, all of them! Most of them became piles of ashes according to the paper, but a few drowned or starved to death (I think the game picks a random death.) When my Sim's child started school I took matters into my own hands. When a new baby was born I had my Sim's husband go (through edit town) and bring the nooboo home for raising until their child birthday, after which they went back home. It was like running an orphanage and kind of fun to see the mixed genetics in Twinbrook. After a while the babies stopped dying and that game file is fine now.
I think for your game, Esther it has to do with some population coding in the game when story progression in on. The game likes to have a certain percentage of houses empty and full, and a mix of toddlers through to elders. I've noticed in my games, when elders die they are either replaced by nooboos being born or more elders moving in. If a death leads to an empty house, someone new moves into town. I think you might be messing with the coding that controls population mix and the game is trying to correct the population explosion by removing some toddlers/kids, because they are the group that is exceeding it's limits.
I'm not trying to discourage you from allowing Mortimer some fun, I'm providing a possible explanation of why the toddlers are drowning.
