Hi,
I just checked FreeBMD and the children you mention were registered in the W.Derby District.. I then serched for others registered in that district, surname Davies and mother's MN Rooney. This brought up Margaret Jun 1912, Charles Mar.1915 and Isabella Dec 1924. Margaret and Isabella appear to have died as infants, but I did not see a likely death for Charles.
Mother's maiden named was not given before 1911, so there is no way of finding others born before that time.. You still need to find the 1911 Census for any others born 1904-1911. I can't do that as I don't have a sub.at the moment. The 1911 Census will also give you the number of children born to the parents and the number that have died., so you will then know if there are any others left to find.
hth and Merry Christmas
Carmela