Thanks every one for your helpful suggestions, I think I have a long job ahead of me, sadly I have no living relatives that can shed some light on things.
avm228 - the  three children that were on the census were 6, 4 and 1 years old so the missing children would have been quite close in age its also possible that there could have been a set of twins. Below is as much information as I have
Henry Tudor Robinson married Harriet Allison July 1903
The children were listed on the census in the following order:
Margaret Louisa daughter, b Marylebone - this name was scribbled out
Henry Herbert son 6, b Marylebone
Frederick Tudor son 4
Violet daughter 1, b Belgravia
At the time of the census the family were living in Peabody Buildings, Pimlico
Thanks again
 Wightlady