Thanks @Akronix ! I think that this is out of the scope for this core checkbox component - I'd like to keep the options as simple possible and this feels like a more specialized UI that would be better suited in a fork (something like CollapsableChecklist).

Another note - once something like plotly/dash-renderer#26 is added, this behaviour will be able to be specified with something like:

dcc.CheckList(options=[
    {
        'value': 'melons',
        'label': html.Details([
            html.Summary('Label of the item'),
            html.Div(html.Img(src='...'), html.Div('More about the mango option')
        ])
    }
])

In other words, we'll be able to allow the user to supply any component as the label, not just strings. In this case, the html.Details component hides and shows content but users could supply any type of label: html.H1, html.Img, etc

Wow, plotly/dash-renderer#26 looks very promising!
And ambitious too. So, until it is done, I'll keep CollapsableChecklist in our "dash components suit: https://github.com/Grasia/grasia-dash-components