Well, this is a bit of a fix.

I tried fitting ten Sims in one house, an adult, a young adult, and 8 toddlers. Problem was, 3 were triplets babies and when I aged them up, two toddlers disappeared.

I kind of flipped out because I had intention of playing with 10. I was worked up. I had set the household limits set to 20 sims.

I was using 5.0 and it also removed the 2 toddlers from the HUD.

Has anybody else come across this error?

Yikes! This is a late reply but the household's after the few 8 disappear if you use the mirror to change sims. From then on, you'd have to use MCC command center to edit sims through "Modify Sim in CAS" command at the sub-menu area.

I have the MCCC mod as well, though I have yet to mess around with the features it has. I will have to look into this more and see if I can find an answer, in the meantime if you get the help needed from someone else I am glad they were able to help you sooner.

Yes. This happens when you have more than 8 sims in your household and the youngest are babies.
I had this happen this weekend. Had 9 Sims, youngest were triplets. 1 disappeared when I had a sim use the mirror to change its appearance. Bringing the total back to 8.

My solution was not to use the mirror options until the babies aged up. Only happens to babies, not toddlers.

If I understood correctly, even the NRaas MC setting of >8 HH members doesn't work if new Sims are added via natural methods (natural birth, marriage, merging Sim with HH), right?

So you can only have more than 8 members by using MC (force baby, add sim to HH), correct?

I had the setting 'more than 8 Sims' set to True in MC and force-added a Sim to the HH for marriage (which brought the HH up to 8 total Sims), then had a Sim get pregnant naturally via Try For Baby. The baby was just born and this squeezed out the oldest family member from the HH. He's not on the Sim list and also disappeared from the relationship panel for the other family members.

I don't want this to happen. I have a save I can revert to in order to prevent this (and maybe move him out), but there's no way I can save him if new Sims are added naturally (without cheats), right?