Hey CGrouse,
Unfortunally, your elder will die when it is her time, no matter if child is in the house, if little one is a toddler or a child, social worker will pick it up and you'll have to chose another family in town to continue playing that neighborhood. Teens will stay playable alone, and game will continue as usual.
If you don't want to risk you have few options:
- move in YA or an adult in the house to make shore kid grow up under your controll
- use one of the options such are life fruit /eat it raw, in a drink, or ambrosia if you can make it
-keep deathflower in your elder inventory, Grim will take it and reset your elder at the beggining of elder age
-not shore if tatoo is still reseting age, or makeover station
-LTR that brings back to previous lifestage
-Cheat edit in Cas or age up that child with birthday cake
-Stop aging in options and age up child with birthday cake until teen
-if you have Generations - use young again potion on your elder
-if you have fontain of youth you can try that (I don't so I don't really know how it works)
Hope one of these will be suitable for your game.