I know this is probably to late to help now but it might help if it happens again to you or someone else. I had a baby sim go missing that I had with a townie (Zelda Mae) and was letting Zelda raise it at her house when I noticed she had come up missing. She still showed in the family tree but was no where to be found in town.
I tried to resetsim to get her back but that didn't work and by the time I noticed she was gone my 4 running saves had been overwritten so I couldn't get her back that way.
She vanished about 4 days ago. Real life days not sim days. But I finally got her back today thanks to twallan's
Overwatch mod.
After opening the Overwatch menu I went under Immediate and chose Recover Missing Sims and now shes back home in Zelda Mae's house.
.
She didn't age any while missing. She's still a Infant even tho her younger sister is now a child. But at least she's back now.