I have a website that I built with genesis and a bought child theme (Mai Theme). The child theme effects the design of the website. The child theme builders are no longer providing updates (only for their new genesis child theme which is a subscription type of payment plan 
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Is there a need to change the child theme for security purposes?
No, child themes don’t usually get updates, so it’s fine.
Thanks, that’s what I was thinking. Though of course they offer for the client to switch to their subscription paying child theme (so that got me second guessing myself)!