It was only as an infant in 1841 that I found her. No record in 1851 or later, so I looked for deaths but there isn't anything.
I'm really sorry to say this but though the Aunt heading off with Matilda is a possibility it's more likely that Matilda didn't survive till 1851. Living conditions in the Gorbals were not good.
My great-great Uncle Thomas died of consumption at the age of 7 - mining family, also Lanarkshire. I knew from the family story he existed, he shows in the 1851 census as a baby and because he died in 1857 there is was a death registered, which I found on SP. It even tells me where he is buried - Wellwynd Churchyard in Airdrie.
No parish records, and believe me I have looked, sat in the Mitchell winding through every page, and no record of the burial because almost certainly it would have been a pauper's grave. As they are now happily building a car park there, no further trace will ever be found. The only route left was the Parish Relief, because there was probably a request for help with the funeral costs - and those record have been lost. I think you have a good chance at the Mitchell for finding those from the Gorbals.
The only reason I know what happened to Thomas is because registration after 1855 compelled it. If he had died on or before 1854 he would have vanished without a trace.
Although you obviously want to go on looking, and good on you, I really do hope you find something, I think there is a possibility that Matilda may have died before 1851, and no record exists for you to find - apart possibly from an application for relief.
I really do hope you prove me wrong!
Maggie 1895