To fix this, you first need to quit without saving or rollback to before the child was taken away. Install MCCC if you have not already.
- Rollback to before the child has been taken away and flag the sim using MCCC to never cull Select Sim > MCCC > Sim Flags > MC Command Center > Flag to Never Cull
- Then allow the child to be taken away and select either playable/freeroaming ghost - it kicks the sim out of the household either way.
- If the child is still visible in the relationship panel of another sim, invite them over that way. If they disappear from the relationship panel, summon them via the mailbox Mailbox > MC Cheats > Sims/NPC Households Cheats > Summon Sim.
- When the child is back you can shift-click with testingcheats on to add them back to the family or use MCCC > Sim Commands > Maintain Sim > Move Sim > Move Into Active Household.
