Very difficult to trace people who were born in 1923 and 1926. Are you positive that they are the children of your James and May. Dorothy probably married years ago, and looking at marriages since about 1940 onwards, there are literally dozens of Dorothy Sims and Arthur Sims getting married. Therefore, you would not know what name to look for in connection with Dorothy, and Arthur could be living anywhere in the British Isles or even abroad.
There was another marriage of a James E Sims to Sarah Pugh.

The marriage of James E. Sims to Sarah Pugh was in Wales. and there are five children, registered in Wales who are called Sims, but have a mother with the maiden name of Pugh.

SIMS Charles C P Wales Radnorshire, Rhayader 1920
SIMS Dorothy Wales Monmouthshire, Newport (Monmouthshire) 1923
SIMS James E P Wales Radnorshire, Rhayader 1924
SIMS Margaret I P Wales Radnorshire, Rhayader 1925
SIMS Arthur Wales Monmouthshire, Newport (Monmouthshire) 1926

All of these children were born in Wales.