I did exactly what you wrote.
Yes, I use a child theme.
In child them I created a folder ‘functions’ and put simplecatch_functions.php file there . Inside I pasted simplecatch_sliders function without a link. I cleaned the cache. Unfortunately, it did not help.
I made the same change in simple-catch theme. I cleaned the cache. Also nothing.
Looks strange, isn’t? Any idea?
UPDATE:
When I enter a change in simple-catch it works. Theoretically child time should be an overriding theme, right?