Thanks for sharing, this is definitely something we can streamline in the future!
In the meantime, you can make use of the drag and drop functionality. If the child cards in ‘People’ are indexed you can drag the card names and drop them onto ‘Investors’ in the sidebar. Equally you can open the ‘People’ card and drag the relevant child cards onto the ‘Investors’ if it is indexed in the sidebar - but then I realise this doesn’t address your particular use case of adding cards to the current parent.
Also quick tip, you can open a card in a new tab by [cmd/ctrl] + [clicking] it’s name. Then if you rearrange the tabs so they are instead two Supernotes windows side by side; drag and drop is functional across these browser windows. So you can drag one card from one browser window to another card on a different browser window.