Travel outside while MN1 is in progress

Hi - I have MN1 application in progress for kids and want to travel to the home country in mid Dec and come back to the UK in around Feb next year.
It has been a month since we have submitted biometrics so hoping to get the registration completed before we fly in Dec, but given that kids does not have a current valid visa (expired due to parents were granted ILR) can they comeback in with their non-visa/visa free (6 months) passport as tourists?

From the MN1 timeline I can see that Home Office has already started processing applications where biometrics was submitted in first week of Oct so I am really hoping it will knock on my door soon.

Thanks

On the top of my head.. the airline/immigration on departure airport would have a problem.

Thanks Aman. That's a really good point, but the passport we hold allows us 6 months free visa on arrival. Wouldn't that be sufficient you think?
There must be some sort of arrangement for kids to travel, what are the options in this case?

Thanks