Hi Paola,
What actually do you want to modify in footer and in header? If you like to change footer credit then its a Pro feature. Yes clean retina supports child theme. You need to first unhook the functions and then only customize the code in your child theme.
Here is an example for ambition theme. You need to do similarly with clean retina theme. If you are unable to do it then you need to hire a developer.
Thank you!
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