so what happens if the police ask to see the kids passports an notice the visa is expired?
just say, "ok, no problem"
read my link
Can Children Be Fined for Overstay?
Children under the age of 15 are not fined for overstaying but will still receive an overstay stamp – if they overstay – when they leave the country.
Please note that being exempt from the overstay fine does not make children exempt from needing a valid visa to enter the country, or a valid extension to stay in the country.