John married Susan Ann Webb in the Dec 1/4 1864. The two surviving children from his first marriage were Mary A Smith aged 6 and Samuel Smith aged 4 according to the 1861 census and I think these children were taken on by Susan as step children.
It mostly sounds right to me, but do be careful with your names, as above in your first posting, it was
Sarah he married in 1864 and
Sarah who was the stepmother! It mixed me up a bit to start with - Smith is bad enough without getting your Sarahs and Susans mixed up

One thing puzzling me, and I admit I haven't looked up all the details, but if Sarah Ann
Webb married William Charles
Clarke in 1856, wouldn't she have been Sarah Ann
Clarke when she married John Smith in 1864? The marriage you mention for then shows John Smith marrying a Sarah
Webb
??
Barbara