The 1939 Register is available on the Find My Past website (subscription or pay-per-view).

So if the family were at the address when the Reg was taken, you should be able to find them.

However, the Reg does not show anyone under 100 years old (i.e. born 1919 or later) unless it has been proved that they have died, and in many cases that may not yet have been done. Such records are obscured by a thick black line.

It is entirely possible that any young children could still be alive.