Wow, that's amazing and fast work!
It was on the 1939 census that I discovered the 'Smart' connection, which was how I got back to find out about Joseph and then subsequently discovered the two girls Elsie and Frances on their Grandparents census in 1911. I didn't know there was a third child called Harold, so that's interesting.
I can't find anything about George and Clara's whereabouts in 1911, when they were clearly together and producing children. It's a mystery what happened to Joseph. I'm assuming he died seeing as the maternal grandparents are housing the children, unless there was some kind of dramatic bust up that separated Clara and Joseph and the grandparents insisted on taking the children. I've imagined all kinds of scenarios in my head! I wondered if he'd been in a mining accident perhaps, but surely then he'd have a death certificate.
I've found who I think Elsie and Frances got married to later on, though there doesn't appear to be many children from them (Frances had a daughter called Margaret).
Thanks for your help and discovering Harold