If you get either thier marriage cert or one of childrens birth certificates it will give her maiden name .

There is a marriage on the FBMD of a Daniel Mackenzie March q. 1845 City of London, there is on the page an Ann Collins but as we not sure where it took place a safer bet would one of the childrens births.
There a two of three possible births for William but this looks like Charlotte
Charlotte Mackenzie Sept q 1849 Bermondsey, Surrey 4 - 41
Charles Mackenzie Sept q. 1856 Greenwich, Kent 1d - 477
Daniel Mackenzie March g. 1854 Bermndsey, Surrey 1d - 71