wp-caption

the site has some html validation errors: https://validator.w3.org/check?uri=http%3A%2F%2Fwww.kidsmoderndesign.co.uk%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

please try to fix these; as long as these errors are present, there is no point looking for the cause of the caption problem.

https://codex.www.ads-software.com/Validating_a_Website

edit:

on second view, there seems to be inline style with the image: style="margin: 5px;" – possibly directly added when editing the image using the ‘advanced’ tab (?)